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Despite subtle variations in soil moisture (SM) across a paddock, irrigation scheduling in New Zealand dairy farms is solely based on the SM monitored at a single location, primarily using an Aquaflex soil moisture sensor at a specified root depth. This study aimed to address this issue by assessing the “effective” root depth of a pasture, calibrating the Aquaflex soil moisture sensor and evaluating the spatial and temporal variability of SM. Twenty non-weighing lysimeters and 1 Aquaflex with 2 sensors installed 125-m away from the lysimeters on the same paddock were utilized for the study. TDR probes with 200-, 500- and 900-mm lengths were installed vertically adjacent to the Aquaflex and the lysimeters for monitoring spatio-temporal variability in SM, and calibrating the Aquaflex. A dry down experiment was performed for investigating the root depth of the pasture. All TDR probes responded to wetting and drying events, with varying SM measurements both vertically and horizontally, due to variations in soil type at different locations, indicating a need of SM monitoring at different locations in the paddock for irrigation scheduling. There was a strong linear relationship between the Aquaflex and TDR probes readings, which can be used to calibrate the Aquflex and improve its reliability for measuring soil moisture and in turn irrigation needs. Over the dry down period, out of the total moisture change in the 0–900-mm soil profile, 96% was contributed by 0–500 mm, indicating that the significant root depth of the pasture lies on the top 500-mm soil profile. Findings of the study can contribute to better irrigation scheduling and to conserve water.

In this study, we investigated the effects of deficit drip irrigation practices on evapotranspiration, water-yield relations, and quality characteristics of two sesame varieties (Muganli-57 and Birkan) grown in the lysimeters under the Mediterranean climate conditions. Irrigation treatments were rainfed (non-irrigated) (I0), drip irrigation with 25% (DI25), 50% (DI50), 75% (DI75), and 100% (DI100) of cumulative evaporation (Epan) measured in Class A pan with three replications. The applied water was corrected by the percentage of vegetation cover until the full cover was attained. Evapotranspiration of Muganli-57 and Birkan ranged from 157.8 to 518.8 mm and 156.4 to 509.2 mm, on average, respectively. The yield values changed between 0.46 and 2.06 t ha−1 in Muganli-57 and between 0.35 and 1.95 t ha−1 in Birkan. Based on the 2-year results, water use efficiency (WUE) of Muganli-57 variety (3.5 kg ha−1 mm−1) was numerically higher than that of the Birkan variety (3.4 kg ha−1 mm−1). In both varieties, it was determined that deficit irrigation generally reduced the yield. However, WUE did not decrease monotonically with decreasing irrigation level. DI50 treatment maximized WUE for two varieties in both years while DI75 maximized WUE for Muganli-57 in 2014. Although irrigation-variety interaction affected relative water content, chlorophyll content index, and seed dry matter, their effect tended to be weak and inconsistent between the 2 years of the study. Deficit irrigation decreased the oil content of sesame and increased its protein content. Similarly, deficit irrigation treatments decreased the linoleic acid content and increased oleic acid content in 1 year but not the other. It was concluded that the Birkan variety is more suitable for irrigated conditions due to the growth characteristics and less saturated fatty acid content. On the contrary, Muganli-57 variety is suitable for non-irrigated conditions in terms of water economy and fatty acid composition.

Surface renewal (SR) is a biometeorological technique that uses high-frequency air temperature measurements above a plant canopy to estimate sensible heat flux. The sensible heat flux is then used to estimate latent heat flux as the residual of a surface energy balance equation. SR previously relied on calibration against other methods (e.g., eddy covariance) to obtain accurate measurements of sensible heat flux, and this need for calibration limited the use of SR to research applications. Our group recently showed that compensating for the frequency response characteristics of SR thermocouples causes the calibration factor to converge near the theoretically predicted value of 0.5 (Shapland et al., Agric For Meteorol 189:36–47, 2014). This led to the development of an inexpensive, stand-alone SR system to measure sensible heat flux without the need for calibration, and here we evaluated the SR system in a mature vineyard containing a weighing lysimeter. Vineyard evapotranspiration (ET) measured with SR was strongly and positively correlated with that from the lysimeter, eddy covariance, and a soil water budget approach. ET measured with the various techniques responded similarly to changes in the microclimatic conditions (i.e., day to day variability) and when water was withheld from the entire vineyard for an extended period. A stress index, calculated using reference and actual ET from SR and lysimetry, was correlated to leaf water potential, stomatal conductance, and volumetric soil water content measurements, but some of these relationships were more variable than others. Our results suggest that the new SR method could potentially be used as a low-cost tool to provide growers with field-specific estimates of crop water use and stress for irrigation management in vineyards.

This study evaluated maize variable bulk surface resistance (rs, s m−1) models developed using field/environmental data from non-irrigated fields in a humid climate. The different rs models were reported in the companion paper. Surface resistance values derived from the application of the new models were inserted in the 1965 Penman–Monteith ET equation to calculate actual maize evapotranspiration (ETa). This is the so-called one-step approach. The evaluation was performed with maize water use data measured with a large weighing lysimeter located in the middle of an irrigated maize field (semi-arid climate) near Bushland, Texas, USA. The evaluation was performed for different time scales (semi-hourly to daily) and the rs models bias (MBE) and root mean square errors (RMSE) were determined. Using daytime 30-min rs models in maize ETa (mm 30-min−1) estimation resulted with the lowest relative error of 15.4%. While daytime average rs models developed from daytime average explanatory variables resulted with the lowest relative error of 20.9% in 30-min maize ETa estimation. When rs models from 30-min and daylight average explanatory variables were used to obtain cumulative daytime maize ETa estimations, resulting errors were lower than for semi-hourly time step. In addition, when daytime 30-min and average rs models were applied in conjunction with a fixed rs nighttime value to obtain daily maize ETa, results were similar than those for the daytime time step. Furthermore, another evaluation incorporated rs values obtained adopted ranges of crop evaporative fraction (EF). When this EF-based rs values were used to estimate maize (ETa, mm 30-min−1) the error found was 5.9 ± 21.7%. This result seems high; however, when the EF range-based rs values were applied in the estimation of maize cumulative daytime ETa, results indicated an error of 5.9 ± 11.9%. In this last case, the relative error was much lower than for 30-min ETa estimations. Therefore, it is concluded that some maize rs models, reported in the companion paper, performed well when evaluated in a different climate and agronomic practices and are suitable for the estimation of ETa using the 1965 Penman–Monteith ET equation. In particular daytime 30-min rs models applied to 30-min intervals of ETa estimation and then accumulated over the day performed better than the others rs models and time steps studied.

Accurate methods to determine irrigation requirements are necessary for the efficient use of water in agriculture. We conducted measurements of transpiration (T) of one almond tree placed in a large weighing lysimeter and instrumented with sap-flow probes for three seasons (2014–2016; 6–9 years after transplanting). We also conducted independent T estimations by the water balance method in four plots. Transpiration was related to reference evapotranspiration (ETo) to obtain the coefficient of transpiration (KT = T/ETo). Average mid-season KT of the lysimeter tree was 0.55, 0.68 and 0.91 in 2014, 2015, and 2016, respectively, and maximum ground cover (GC) was 55, 59 and 55% for the same years. These KT values standardized by GC were supported by the independent estimations of KT/GC obtained in the plots using the water balance, except in 2016, when a very high KT value was observed in the lysimeter tree. There were significant fluctuations in daily KT during mid-season, which we found to be related to wind speed. Furthermore, the exceptionally high KT/GC relationship of 2016 was apparently related to the very high crop load in the lysimeter of that year (75% more than the 2 other normal years). Hourly bulk canopy conductance values were derived from lysimeter T records to confirm the high transpiration rates prior to harvest during 2016. From the KT values measured here, we propose that the mid-season KT of fully mature almond orchards, with a GC of 75%, should be around—between 0.9 and 1.05. An estimation of evaporation from soil, which is dependent on the method of irrigation, needs to be added to obtain the net water requirements.

Accurate parameterization of reference evapotranspiration (ET0) is necessary for optimizing irrigation scheduling and avoiding costs associated with over-irrigation (water expense, loss of water productivity, energy costs, and pollution) or with under-irrigation (crop stress and suboptimal yields or quality). ET0 is often estimated using the FAO-56 method with meteorological data gathered over a reference surface, usually short grass. However, the density of suitable ET0 stations is often low relative to the microclimatic variability of many arid and semi-arid regions, leading to a potentially inaccurate ET0 for irrigation scheduling. In this study, we investigated multiple ET0 products from six meteorological stations, a satellite ET0 product, and integration (merger) of two stations’ data in Southern California, USA. We evaluated ET0 against lysimetric ET observations from two lysimeter systems (weighing and volumetric) and two crops (wine grapes and Jerusalem artichoke) by calculating crop ET (ETc) using crop coefficients for the lysimetric crops with the different ET0. ETc calculated with ET0 products that incorporated field-specific wind speed had closer agreement with lysimetric ET, with RMSE reduced by 36 and 45% for grape and Jerusalem artichoke, respectively, with on-field anemometer data compared to wind data from the nearest station. The results indicate the potential importance of on-site meteorological sensors for ET0 parameterization; particularly where microclimates are highly variable and/or irrigation water is expensive or scarce.

Expansion of irrigated olives, combined with prevalence of water containing dissolved salts, leads to irrigation-induced exposure of olive trees to salinity. Root zone salinity can rise either as a function of increasing input irrigation water salinity (ECi) or relative reductions in water for leaching. We investigated five ECi levels from 0.5 to 11.0 dS m−1 with a constant leaching fraction (LF) of 0.29 and five leaching levels from 0.05 to 0.44 (drainage: irrigation ratio) with water of ECi = 5.0 dS m−1 on young bearing Olea europaea cv Barnea olive trees grown in 2.5 m3 weighing-drainage lysimeters over three years. Tree-scale response to increased salinity did not demonstrate any sign of a threshold value and was not differentiated by the cause of salinity, be it changes in input irrigation water salt concentrations or changes in LF. Soil salinity, measured as electrical conductivity of saturated soil paste extract (ECe) and maintained at stable levels over time, decreased tree water consumption and tree size measured as trunk area or above-ground biomass by 40–60 % as it increased from 1.2 to around 3.5–4.0 dS m−1. Further increases in ECe to as high as 7.5 dS m−1 brought these parameters to 20–30 % of the treatment with low salinity. Fruit yield also decreased with increasing salinity, albeit with less drastic relative effects. An analytical model calculating water and salt balance and subsequent evapotranspiration or biomass as a function of irrigation water quantity and salinity successfully predicted the measured results.

A common irrigation strategy is to replenish the soil water reservoir according to evapotranspiration (ET). However, the ET from plants under deficit irrigation is not well explored and is normally assumed to be a constant fraction of their respective well-watered condition. In the current experiment, we hypothesized that the ratio between the ET of well-watered (WW) and water deficit (WD) grapevines is not constant but depends on the reference ET (ETO). The hypothesis was tested using lysimeters over two consecutive seasons to measure the ET of WD grapevines (Vitis vinifera L.) that were irrigated at 35 % of a second set of lysimeters with WW vines. The WD treatment started at veraison and resulted in a quick depletion of the soil water reservoir; thereafter a relatively stable soil water content (θ) and crop coefficient (KC) were measured. The ET of the WD vines was 20–75 % lower than that of the WW vines, depending on the reference evapotranspiration (ETO). Under high ETO, the difference between the treatments was much larger than under low ETO. The dynamic ratio between the ET of the treatments demonstrates the difficulty in predicting the ET of WD plants and suggests that irrigation according to a constant fraction from a WW plant might result in either excessive or insufficient irrigation amounts. The high correlation between instantaneous stomatal conductance (g s) measurements and KC emphasizes the advantage of utilizing g s to improve current irrigation models.

The use of overhead trellis systems for the production of dry-on-vine (DOV) raisins and table grapes in California is expanding. Studies were conducted from 2006 to 2009 using Thompson Seedless grapevines grown in a weighing lysimeter trained to an overhead arbor trellis and farmed as DOV raisins for the first two years and for use as table grapes thereafter. Maximum canopy coverage for the two lysimeter vines across years was in excess of 80 %. Seasonal (15 March–31 October) evapotranspiration for the lysimeter vines (ETLys) was 952 mm in 2007 (farmed as DOV raisins) and 943 and 952 mm (when farmed as table grapes). The maximum crop coefficient (K cLys) across all 4 years ranged from 1.3 to 1.4. These maximum values were similar to those estimated using the relationship where K c is a function of the amount of shaded area measured beneath the canopy at solar noon (K c = 0.017 × percent shaded area). Covering the lysimeter’s soil surface with plastic (and then removing it) numerous times during the 2009 growing season (1 June–14 September) reduced ETLys from an average of 6.4 to 5.6 mm day−1 and the K c from 1.07 to 0.93. A seasonal basal K c (K cb) was calculated for grapevines using an overhead trellis system with a 13 % reduction in the K cLys across the growing season.

New cultivars of sorghum for biomass energy production are currently available. This crop has a positive energy balance being irrigation water the largest energy consumer during the growing cycle. Thence, it is important to know the biomass sorghum water requirements, in order to minimize irrigation losses, thus saving water and energy. The objective of this study was to quantify the water use and crop coefficients of irrigated biomass sorghum without soil water limitations during two growing seasons. A weighing lysimeter located in Albacete (Central Spain) was used to measure the daily biomass sorghum evapotranspiration (ETc) throughout the growing season under sprinkler irrigation. Seasonal lysimeter ETc was 721 mm in 2007 and 691 mm in 2010. The 4 % higher ETc value in 2007 was due to an 8 % higher evaporative demand in that year. Maximum average K c values of 1.17 in 2007 and 1.21 in 2010 were reached during the mid-season stage. The average K c values for the 2 years of study were K c-ini: 0.64 and K c-mid: 1.19. The seasonal evaporation component was estimated to be about 18 % of ETc. The average basal K c (K cb) values for the two study years were K cb-ini: 0.11 and K cb-mid: 1.17. The good linear relationship found between K cb values and the fraction of ground cover (f c) and the excellent agreement found between Normalized Difference Vegetation Index and different biophysical parameters, such as K cb and f c, will allow monitoring and estimating the spatially distributed water requirements of biomass sorghum at field and regional scales.

Wheat is one of the crops occupying the largest areas in the world (218 million ha in 2013). Understanding the land–atmosphere energy exchanges over these croplands becomes important not only for agronomy but also for climatic and meteorological aspects. This study continues previous work on the estimation of actual evapotranspiration (ET) and the assessment of crop coefficients of sorghum, sunflower, or canola. Two variations of a simple two-source energy balance (STSEB) approach were used in combination with land surface temperature measurements to calculate hourly and daily values of surface fluxes and actual ET. An experiment was carried out during the spring season of 2014 in Las Tiesas experimental farm in Barrax, Spain. Soil and canopy temperature components together with meteorological variables and biophysical parameters were measured from planting to senescence. Comparison to lysimeter measurements showed calculation errors of ±0.11 mm h−1 and ±0.8 mm day−1 for hourly and daily ET values, respectively, whereas an underestimation no >4 % resulted from the entire campaign. Partition between soil and canopy components yielded a ratio of evaporation (E) to transpiration (T) of 36–64 %, respectively, for the total growing season. Dual crop coefficients were also calculated and compared to those proposed by FAO-56. Although separate E and T measurements were not available, similar results between the STSEB and FAO-56 models demonstrate the utility of the STSEB for investigating management strategies aimed at increasing crop water use efficiency.

Crop evapotranspiration (ETc) and crop coefficient (K c) for drip-irrigated bell pepper grown on plastic-mulched beds for climatically different growing seasons were quantified using drainage lysimeters located in the Southwest Florida. We tested whether: (1) the literature K c (with climatic adjustment) yields accurate ETc estimates; (2) K c values are applicable across the growing seasons that are climatically different; and (3) a thermal-based generic model can be developed to accurately predict K c for gradually cooling (fall) and warming (spring) seasons. Spring ETc (229 mm) was statistically (p = 0.03) higher than fall (189 mm). Seasonal K c values were also statistically different, and average fall value (0.86) was 20 % higher than spring (0.72), confirming K c’s seasonality. The K c values for the fall were statistically higher than FAO-56 K c’s which underestimated ETc by 22 %. Use of fall K c overestimated spring ETc by 24 %, while it was 12 % underestimation for the opposite. These errors were 16–69 % of rainfall and 26–46 % of farm-scale surface flows highlighting the need for accurate ETc predictions. The seasonal differences in K c could be accurately predicted with a generic growing degree days (GDD) model (r 2 = 0.95). The results not only provide an improved ETc estimate for plastic-mulched pepper in the study area, but also for other regions using the GDD-based model that utilize readily available temperature data. The GDD model can be further improved using additional data in future.

Increased water scarcity demands more efficient use of water in the agricultural sector which is the primary consumer of water. Precise determination of irrigation requirements based on specific crop parameters is needed for accurate water applications. We conducted a 4-year study on almond evapotranspiration using a large weighing lysimeter. Tree canopies changed from 3 to 48 % ground cover during the course of the study. Sap flow measurements made on the lysimeter tree provided a continuous record of tree transpiration. We propose to use the daily fraction of photosynthetically active radiation intercepted by the canopy (fIRd) as a predictor of almond orchard maximum transpiration. The transpiration coefficient (T/ETo or KT) was related to the fIRd of the last two years, and the ratio between fIRd and KT stayed more or less constant around a value of 1.2. Such value extrapolated to the size of a mature orchard with 85 % intercepted radiation gives a KT of around 1.0, a number above the standard recommendations, but fully compatible with the maximum Kc values of 1.1–1.15 recently reported., M Espadafor is a recipient of research fellowship BES-2010-033883 from the Spanish Ministry of Science and Innovation. Financial support from the Spanish Ministry of Science and Innovation (AGL2009-07350) is also acknowledged.

Following online publication of the paper, the authors have detected that the slope of the lines of linear regressions shown in the text

In semi-arid environments, the use of irrigation is necessary for sunflower production to reach its maximum potential. The aim of this study was to quantify the consumptive water use and crop coefficients of irrigated sunflower (Helianthus annuus L.) without soil water limitations during two growing seasons. The experimental work was conducted in the lysimeter facilities located in Albacete (Central Spain). A weighing lysimeter with an overall resolution of 250 g was used to measure the daily sunflower evapotranspiration throughout the growing season under sprinkler irrigation. The lysimeter container was 2.3 m × 2.7 m × 1.7 m deep, with an approximate total weight of 14.5 Mg. Daily ET c values were calculated as the difference between lysimeter mass losses and lysimeter mass gains divided by the lysimeter area. In the lysimeter, sprinkler irrigation was applied to replace cumulative ET c, thus maintaining non-limiting soil water conditions. Seasonal lysimeter ET c was 619 mm in 2009 and 576 mm in 2011. The higher ET c value in 2009 was due to earlier planting and a longer growing season with the maximum cover coinciding with the maximum ET o period. For the two study years, maximum average K c values reached values of approximately 1.10 and 1.20, respectively, during mid-season stage and coincided with maximum ground cover values of 75 and 88 %, respectively. The dual crop coefficient approach was used to separate crop transpiration (K cb) from soil evaporation (K e). As the crop canopy expanded, K cb values increased while the K e values decreased. The seasonal evaporation component was estimated to be about 25 % of ET c. Linear relationships were found between the lysimeter K cb and the canopy ground cover (f c) for the each season, and a single relationship that related K cb to growing degree-days was established allowing extrapolation of our results to other environments.

49 Pags., 3 Tabls., 6 Figs. The definitive version is available at: http://link.springer.com/journal/271, Gross sprinkler evaporation losses (SELg) can be large and decrease irrigation application efficiency. However, it is not universally established how much of the SELg contributes to decrease the crop evapotranspiration during the sprinkler irrigation and how much are the net sprinkler losses (SELn). The components of SEL were the wind drift and evaporation losses (WDEL) and the water intercepted by the crop (IL). The gross WDEL (WDELg) and evapotranspiration (ET) were measured simultaneously in two alfalfa (Medicago sativa L.) plots, one being irrigated (moist, MT) and the other one not being irrigated (dry, DT). Catch can measurements, mass gains and losses in the lysimeters and micrometeorological measurements were performed to establish net WDEL (WDELn) during the irrigation and net IL (ILn) after the irrigation as the difference between ETMT and ETDT. Also, equations to estimate ILn and net sprinkler evaporation losses (SELn) were developed. ILn was strongly related to vapor pressure deficit (VPD). SELn were 8.3% of the total applied water. During daytime irrigations, SELn was 9.8% of the irrigation water and slightly less than WDELg (10.9%). During nighttime irrigations SELn were slightly greater than WDELg (5.4% and 3.7%, respectively). SELn was mainly a function of wind speed., This research was funded by the MCINN of the Government of Spain through grants AGL2007-66716-C03-01/02, AGL2010-21681-C03-01/03; the European Commission through grant QUALIWATER (INCO-CT-2005-015031) and by the FPI-MINECO PhD grant program.

To reveal the influence of non-flooding controlled irrigation (NFI) on gaseous nitrogen (N) losses in forms of ammonia volatilization (AV) and nitrous oxide (N2O) emissions from high N inputs rice paddies, lysimeter experiments were conducted with flooding irrigation (FI) as check. Compared with FI paddies, AV losses in NFI paddies decreased by 18.5–20.5 % and N2O emissions increased by 1.43–1.9 kg N ha−1. Weekly AV losses immediately after fertilization accounted for over 83 % of seasonal losses in both treatments. High N2O emissions from NFI paddies always occurred in drying process after N application, with peaks observed when water-filled pore space (WFPS) fell in 75–85 %. Water management immediately after N fertilization is crucial for mitigating gaseous N losses from rice paddies. Bringing N into shallow rhizosphere by irrigation and covering it with deep water will be helpful in preventing AV. Maintaining a flooding period and keeping WFPS higher than 85 % in the first drying process after fertilization might be effective to reduce N2O emissions peaks for NFI paddies.

Estimation of crop evapotranspiration (ETC) for certain crops such as potato is very important for irri- gation planning, irrigation scheduling and irrigation sys- tems management. The primary focus of this study was to investigate the accuracy of the adaptive neurofuzzy infer- ence system (ANFIS) and support vector machines (SVM) for potato ETC estimation when lysimeter measurements or the complete weather data for applying the FAO method are not available. The estimates of the ANFIS and SVM models were compared with the empirical equations of Blaney-Criddle, Makkink, Turc, Priestley-Taylor, Har- greaves and Ritchie. The performances of the different SVM and ANFIS models were evaluated by comparing the corresponding values of root mean square error (RMSE), mean absolute error (MAE) and correlation coefficient (r). The drawn conclusions confirmed that the SVM and AN- FIS models could provide more accurate ETC estimates than the empirical equations. Overall, the minimum RMSE (0.042 mm/day) and MAE (0.031 mm/day) values and the maximum r value (0.98) were obtained using the SVM model with mean air temperature, relative humidity, solar radiation, sunshine hours and wind speed as inputs.

Use of literature crop coefficient (Kc) values for quantifying evapotranspiration (ETc) under non-standard conditions such as plastic mulch, shallow water table, and sub-tropical conditions can lead to inaccurate ETc estimates. A 5-year experiment was conducted for fall crop growing seasons in south Florida to quantity bi-weekly ETc and Kc for bell pepper grown under shallow water table and plastic mulch environments using large drainage lysimeters. The ETc values varied from 205 to 320 mm with a seasonal average of 267 mm. Average Kc values for bell pepper for development, mid-season, and late stages were 1.05, 1.21, and 1.28, respectively. Higher than literature initial Kc values were due to rainfall and use of sub-irrigation system to maintain artificially high water table which results in high soil moisture in the bare soil area—such high moisture results in high evaporation. The Kc values from this study were statistically higher than literature values. Use of literature Kc values resulted in underestimating ETc by 27–37%. The Kc values would provide improved estimates of sub-irrigated pepper ETc in subtropical Florida and elsewhere with similar environment.

Accurate prediction of crop coefficient (K c) is necessary for proper irrigation management. We explored CropSyst for determining irrigation requirements of apple trees and for accuracy of K c prediction. Values of K c were compared to those obtained, over 2002–2010, from lysimeter-grown trees. Over these years, trees had different ratios of height (H) to width (W). CropSyst predicted irrigation requirements using tree light interception and water uptake sub-model components. Parameters of the model were adjusted using data obtained from the lysimeter in 2010. Tree light interception sub-model was verified by 2007 data. After parameterization, good agreement was found between simulated and measured K c over different seasons. The porosity coefficient of the canopy was related to changes in tree’s H/W ratio and leaf overlapping. Accordingly, different porosity values could be estimated for each year. When yearly changes in canopy porosity was considered, CropSyst improved K c prediction and generated relevant information for managing irrigation under changing canopy shape for apple trees.

We hypothesized that organic matter (OM) content originating from treated wastewater (TWW) irrigation and soil texture dominate the intensity of soil water repellency. The relationship between soil texture, wastewater treatment level, and water repellency was examined in a 3-year lysimeter experiment (2008–2010). Soil type–water quality combinations, consisting of three soils with different specific surface area (SSA) and four levels of water quality differing in OM content, were tested. In each year, water repellency developed in all TWW quality treatments, but not in freshwater-irrigated controls. At the end of each year (except 2009), the highest degree of repellency was exhibited by sandy soil treated with the lowest quality TWW (highest OM content). The lowest degree of water repellency was consistently exhibited by the soil with the highest SSA irrigated with the highest quality TWW (lowest OM content). Water quality, rather than SSA, was the dominant factor in determining degree of repellency induced by TWW irrigation.

Prediction of plant water status is necessary for the judicious application of regulated deficit irrigation. CropSyst, a generic crop growth model that is applicable to fruit trees, was used to forecast plant water potential for irrigation management recommendations in a pear orchard. Plant water potential is predicted along with tree transpiration using Ohm’s law analogy. The parameters of the model were adjusted by using field measured data on a lysimeter-grown pear tree. After adjustment, and using the same lysimeter data, a satisfactory agreement was found between simulated and measured tree transpiration, light interception, and stem water potential. Model simulations were also performed for other independent field data. These corresponded to eight different conditions of a deficit-irrigated field experiment in a pear orchard. Each condition differed in soil texture, time of irrigation cut-off, crop load, and tree leaf area. Deficit irrigation was managed first by withholding irrigation until reaching a threshold in midday stem water potential of −1.5 MPa. Subsequently, irrigation was applied at fixed proportions of full irrigation requirements. Simulations with CropSyst were used as decision support system that could work independently of stem water potential measurements. Simulations in all eight sites were satisfactory at providing adequate time without irrigation during the first part of the deficit period. A highly significant relationship (r 2 = 0.71) between predicted and measured stem water potentials was found for a simulation period of 40 days. Simulations for longer periods (i.e. 74 days) decreased the r 2 to 0.61, and for this reason after resuming irrigation, slight deviations were found for the average stem water potential in two out of five sites. In conclusion, CropSyst produced relevant information for managing deficit irrigation in simulation periods shorter than 40 days.

Identification of drainage water allows assessing the effectiveness of water management. Passive capillary wick-type lysimeters (PCAPs) were used to monitor water flux leached below the root zone under an irrigated cropping system. Wireless lysimeters were developed for web-based real-time online monitoring of drainage water using a distributed wireless sensor network (WSN). Twelve PCAP sensing stations were installed across the field at 90 cm below the soil surface, and each station measured the amount of drainage water using two tipping buckets mounted in the lysimeter and continually monitored soil water contents using two soil moisture sensors installed above the lysimeter. A weather station was included in the WSN to measure micrometeorological field conditions. All in-field sensory data were periodically sampled and wirelessly transmitted to a base station that was bridged to a web server for broadcasting the data on the internet. Communication signals from the in-field sensing stations to the base station were successfully interfaced using low-cost Bluetooth wireless radio communication. Field experiments resulted in high correlation between estimated and actual drainage with r 2 = 0.95 and confirmed a reliable wireless communication throughout the growing season. A web-linked WSN system provided convenient remote online access to monitor drainage water flux and field conditions without the need for costly time-consuming supportive operations.

Two-year lysimeter experiments were conducted to determine groundwater contributions by safflower (Carthamus tinctorius L.) crop. The plants were grown in twenty columns each with a diameter of 0.40 m packed with Silty Clay soil. The experiments were carried out in a complete randomized blocks design with four replicates. In each experiment, five treatments were applied by maintaining groundwater salinity to a control treatment with EC 1 dS/m, while the groundwater salinity of the other treatments was 2, 5, 8 and 10 dS/m, and 0.8 m water table level, respectively. The use of groundwater as a part of crop evapotranspiration was characterized by using daily measurements of the water level in Mariotte tubes. The extra magnitude of irrigation water requirement for each treatment was applied by water with EC of 1 dS/m. The results of experiments showed that for different control treatments with 1 dS/m, 2, 5, 8 and 10 dS/m, the groundwater contributions were achieved as 59, 51, 38, 32 and 19% of the total plant water requirements, respectively.

A five-year experiment (2002–2006) was conducted to determine apple (cv ‘Golden Smoothee’) and pear (cv ‘Conference’) crop coefficients (Kc) using two large weighing lysimeters. Daily reference evapotranspiration (ETo) and crop evapotranspiration (ETc) were evaluated. Midday canopy light interception of both crops planted in hedgerows, 4 × 1.6 m, was determined on a weekly basis from bud-break until leaf fall from year 2002 (fourth after planting) to year 2006 of both plantations. Relationships between canopy light interception and calculated Kc (ETc/ETo) were evaluated from bud-break until harvest. There were differences in Kc values between apple and pear trees. When daily Kc values from bud-break until harvest were adjusted to hyperbolic functions each year, adjusted curves for pear trees were very similar regardless of year (maximum Kc around 1.0). In apple trees, the maximum values of Kc increased over time, from 0.49 in 2002 to 1.04 in 2006. Midday light interception in both apple and pear trees increased during the 5 years of experiment from 29.0 to 45.6% in apples and from 27.5 to 41.6% in pears in midsummer. Although there was a significant positive correlation between midday canopy light interception and Kc in apple and pear trees, in different times within a specific year, these relationships were different between crops. While the apple data fitted into the same equation regardless of the year, different equations were needed to fit the pear data in different years. This discrepancy may have been related to differences in the canopy properties between apple and pear trees. Pear canopies had higher porosity than apple canopies and thus improved light penetration. Apple trees were more vigorous and produced taller and denser canopies. Pear Kc values were greatly influenced by the evaporative demands of different years and consequently differences in midday canopy light interception did not adequately reflect the differences in Kc across the two species.

Eddy covariance (EC) systems are being used to assess the accuracy of remote sensing methods in mapping surface sensible and latent heat fluxes and evapotranspiration (ET) from local to regional scales, and in crop coefficient development. Therefore, the objective was to evaluate the accuracy of EC systems in measuring sensible heat (H) and latent heat (LE) fluxes. For this purpose, two EC systems were installed near large monolithic weighing lysimeters, on irrigated cotton fields in the Texas High Plains, during the months of June and July 2008. Sensible and latent heat fluxes were underestimated with an average error of about 30%. Most of the errors were from nocturnal measurements. Energy balance (EB) closure was 73.2–78.0% for daytime fluxes. Thus, daylight fluxes were adjusted for lack of EB closure using the Bowen ratio/preservation of energy principle, which improved the resulting EC heat flux agreement with lysimetric values. Further adjustments to EC-based ET included nighttime ET (composite) incorporation, and the use of ‘heat flux source area’ (footprint) functions to compensate ET when the footprint expanded beyond the crop field boundary. As a result, ET values remarkably matched lysimetric ET values, with a ‘mean bias error ± root mean square error’ of −0.03 ± 0.5 mm day−1 (or −0.6 ± 10.2%).

Vegetative growth and water relations of Thompson Seedless grapevines in response to applied water amounts at various fractions of measured grapevine ETc were quantified. Treatments ranged from no applied water up to 1.4 times the water used by vines growing in a weighing lysimeter. All treatments were irrigated at the same frequency as the vines in the lysimeter (whenever they used 2 mm of water), albeit at their respective fraction. Soil water content and midday leaf water potential (Ψl) were measured routinely in four of the irrigation treatments across years. The amount of water depleted in the soil profile ranged from 190 mm for the 0.2 treatment in 1993 to no water depletion for the 1.4 treatment in 1992. The irrigation treatments significantly affected midday Ψl, total shoot length, leaf area per vine, pruning weights and trunk diameter; as applied water decreased so did vegetative growth. Pruning weights were a linear function of the seasonal, mean midday Ψl across growing seasons. The application of water amounts in excess of evapotranspiration negatively affected vegetative growth some of the years. A companion paper will demonstrate that over-irrigation can negatively affect reproductive growth of this grape cultivar due to excess vegetative growth.

Efficient water use through improved irrigation scheduling is expected to moderate fast declining groundwater levels and improve sustainability of the Ogallala Aquifer. An accurate estimation of spatial actual evapotranspiration (ET) is needed for this purpose. Therefore, during 2007, the Bushland ET and Agricultural Remote Sensing Experiment (BEAREX07) was conducted at the USDA-ARS, in Bushland, Texas, to evaluate remote sensing (RS)-based surface energy balance models. Very high-resolution aircraft images were acquired using the Utah State University airborne multispectral system. Instantaneous ET was estimated using a two-source energy balance model (TSM). A minor modification was made in the calculation of sensible heat fluxes to improve ET estimation. Furthermore, a sensitivity analysis of selected variables was conducted to evaluate their effect on ET estimation. Data from four weighing lysimeters, planted to sorghum and corn, were used for evaluating ET predictions. Instantaneous ET was predicted with mean bias error and root mean square error of 0.03 and 0.07 mm h−1 (4.3 and 11.7%), respectively. Results indicated that crop height, roughness length for momentum transfer, clumping factor and soil resistance sub-models need to be refined. Nevertheless, the application of the TSM using high-resolution RS imagery in the Southern High Plains is promising.

A lysimeter experiment was conducted to investigate the effect of water table management (WTM) on distribution of soil salinity and annual alfalfa (Medicago scutellata) yield. Subirrigations with three levels of water table namely, 0.5 (WT0.5), 0.7 (WT0.7), and 1.0 m (WT1.0) and a free drainage (FD) conventional irrigation treatment were selected for this study. All treatments were arranged in a complete randomized block design with three replicates. The results of this study indicated that the average soil electrical conductivity of the saturated extract (ECe) in the root zone gradually increased and exceeded the designated crop threshold value (4 dS/m) after the first forage harvest in subirrigated lysimeters. A higher salt accumulation was observed at the WT0.5 treatment. The average dry matter yield of annual alfalfa in WT0.5 and WT0.7 treatments was found to be 52 and 73% higher compared with the control treatment, respectively.

Weighing lysimeters are the standard method for directly measuring evapotranspiration (ET). This paper discusses the construction, installation, and performance of two (1.52 m × 1.52 m × 2.13-m deep) repacked weighing lysimeters for measuring ET of corn and soybean in West Central Nebraska. The cost of constructing and installing each lysimeter was approximately US $12,500, which could vary depending on the availability and cost of equipment and labor. The resolution of the lysimeters was 0.0001 mV V −1 , which was limited by the data processing and storage resolution of the datalogger. This resolution was equivalent to 0.064 and 0.078 mm of ET for the north and south lysimeters, respectively. Since the percent measurement error decreases with the magnitude of the ET measured, this resolution is adequate for measuring ET for daily and longer periods, but not for shorter time steps. This resolution would result in measurement errors of less than 5% for measuring ET values of ≥3 mm, but the percent error rapidly increases for lower ET values. The resolution of the lysimeters could potentially be improved by choosing a datalogger that could process and store data with a higher resolution than the one used in this study.

The expansion of permanent trickle irrigation systems in Sao Paulo (Brazil) citrus has changed the focus of irrigation scheduling from determining irrigation timing to quantifying irrigation amounts. The water requirements of citrus orchards are difficult to estimate, since they are influenced by heterogeneous factors such as age, planting density and irrigation system. In this study, we estimated the water requirements of young ‘Tahiti’ lime orchards, considering the independent contributions from soil evaporation and crop transpiration by splitting the crop coefficient (Kc = ETc/ETo) into two separate coefficients; Ke, a soil evaporation coefficient and Kcb, a crop transpiration coefficient. Hence, the water requirement in young ‘Tahiti’ lime (ETy) is ETy = (Ke + Kcb) · ETo, where ETo is the reference crop evapotranspiration. Mature tree water requirement (ETm) is ETm = Kcb · ETo, assuming no soil water evaporation. Two lysimeters were used; one was 1.6 m in diameter and 0.7 m deep, and the other was 2.7 m in diameter and 0.8-m deep. The first one was used to calculate evaporation and the second one was used for transpiration. ETo was estimated by the Penman–Monteith method (FAO-56). The measurements were conducted during a period between August 2002 and April 2005 in Piracicaba, Sao Paulo state, Brazil. The lysimeters were installed at the center of a 1.0-ha plot planted with ‘Tahiti’ lime trees grafted on ‘Swingle’ citrumelo rootstock. The trees were 1-year old at planting, spaced 7 × 4 m, and were irrigated by a drip irrigation system. During the study period, Kc varied between 0.6 and 1.22, and Kcb varied between 0.4 and 1.0. The results suggested that for young lime trees, the volume of water per tree calculated by Ke + Kcb is about 80% higher than the volume calculated using Kc. For mature trees, the volume of water per tree calculated using just Kcb can be 10% less than using Kc. The independent influence of soil evaporation and transpiration is important to better understand the water consumption of young lime trees during growth compared to mature lime trees.

Despite the steadily increasing area under protected agriculture there is a current lack of knowledge about the effects of the 30% black shading screen on microclimate and crop water requirements. Meteorological and lysimeter measurements inside a screenhouse planted with sweet pepper were compared to external reference data. Irrigation water use efficiency (IWUE) was calculated from yield records Y and water meter readings I applied. Shading reduced mean global radiation R G by more than 40%, and the screen transmissivity τ screen was shown to vary with solar elevation angle β. Wind speed inside the screenhouse u in was reduced by more than 50%. Crop water requirements ET c were 38% lower than estimates for an open field crop, suggesting a significant water saving potential when using screenhouses. However, the screen did not significantly modify maximum temperature T max and daily vapor pressure deficit. The FAO-Penman–Monteith approach based on meteorological measurements in the screenhouse accurately predicted daily crop evapotranspiration, and was in close agreement with lysimeter measurements. IWUE was relatively high (10.7 kg m−3 in 2004 and 13.5 kg m−3 in 2005), but additional research is required to quantify the effect of shading on yield as well as to determine the water saving potential of other commonly used screens.

Based on evaporation from a 20 cm diam- eter pan placed above the crop canopy, sprinkler irrigation scheduling of winter wheat was studied in the North China Plain (NCP) in the 2001-2004 winter wheat seasons. Results showed that pan evaporation (Epan,C) was closely related to actual evapotranspira- tion (ET) measured using weighing lysimeters. The combined pan-crop coefficient (Kc,pan), the ratio of ET to Epan,C, was closely related to leaf area index (LAI ) and plant height. Data from the 2002-2003 season were used to establish the relationships be- tween Kc,pan and LAI (method A) or plant height (method B), and used to determine the crop coeffi- cient (method C). ET computed by the three methods was compared with measured ET using lysimeters in the 2001-2002 and 2003-2004 seasons. Mean relative error of estimated daily ET by the three methods ranged from 20 to 30%, and the relative error in cumulative ET in the experimental periods ranged from 1 to 19%. Among the three methods, results from methods A and B were not significantly different from each other (P > 0.01), and were closer to the lysimeter data than results from method C (P < 0.001). Method B, being easier to measure, was recommended for ET estimation in NCP.

The Bowen Ratio-Energy Balance (BREB) is an accurate method often used to measure the latent heat flux (λE) due to its simplicity and portability. However, its performance in advective areas is less clear and its accuracy may depend on the equality of eddy transfer coefficients for heat and water vapor. In this work, hourly measured λE of a reference crop (Festuca arundinacea Schreb.) using a BREB system was compared with lysimeter-measured λE under moderate to severe advective conditions. The lysimeter resolution for hourly records was 22.6 W m−2. The analysis of the eddy transfer coefficients was made using simultaneous measurements of fluxes and vertical gradients of temperature and humidity. To avoid computational problems when β→ −1, some hourly periods were discarded in the analysis. Rejected data amounted to 37% of the total, although the cumulative evapotranspiration (ET) during these hours did not exceed 13% of the total ET. The BREB method overestimated daily ET by an average of 5.5% and by 5.7% when only daylight hours were considered. Under stable atmospheric conditions the method was less accurate, with relative errors of 21% vs. 11% under unstable conditions. For daylight hours, accuracy was higher under unstable conditions (RMSE = 36.15 W m−2) than under stable conditions (RMSE = 50.20 W m−2), which had larger overestimations of ET (6.3 vs. 5.1%). The main source of error appears to come from insufficient fetch resulting in local advective conditions. Nevertheless, and from a purely practical perspective, under the advective conditions of these measurements the BREB technique provides accurate ET fluxes with limited errors.

Irrigation frequency is one of the most important factors in drip irrigation scheduling, and a proper irrigation frequency can establish moderate moist and oxygen conditions in the root zone throughout the crop period. Field experiments on the effects of irrigation frequency on radish growth and water use were carried out in 2001 and 2002. The experiment included six irrigation frequencies: once every day, once every 2 days, once every 3 days, once every 4 days, once every 6 days and once every 8 days. There was no significant difference among the six treatments on radish development and yield, but significant differences in radish roots distribution and market quality were found. Radishes irrigated once every 3 days had well-developed roots throughout the crop period, the lowest cracking rate and the least number of radishes of Grade 3. The observation results of lysimeter in 2002 showed that radish evapotranspiration decreased as irrigation frequency decreased, and the general changing tendency of 2-day ET of high irrigation frequency was related to that of 2-day evaporation. It is recommended that radish irrigation frequency should be once every 3 days and the irrigation amount should be estimated according to the evaporation of 20 cm diameter pan in the North China Plain.

A 4-year experiment was conducted using a large weighing lysimeter to determine the crop coefficient and crop water use of a late-season peach cultivar (Prunus persica (L.) Batsch, cultivar O'Henry) irrigated with a surface drip system. Two trees were planted in a 2×4×2 m deep weighing lysimeter that was surface irrigated with ten 2 L/h in-line drip emitters spaced evenly around the trees. Irrigation was applied in 12 mm applications after a 12 mm water loss threshold was exceeded as measured by the lysimeter. The crop coefficient (Kc) was calculated using the measured water losses and grass reference evapotranspiration calculated using the CIMIS Penman equation. Kc was plotted against day of the year and linear, quadratic, and cubic regressions were fitted to the data. A three-segment linear and the cubic equation had the best fit to the data. The maximum Kc determined for the linear fit in this experiment was 1.06 compared with a maximum of 0.92 recommended for use in California and 0.98 calculated using the FAO method. Average annual water use for the 4 years of the experiment was 1,034 mm. Mid-day canopy light interception was found to be well correlated with the crop coefficient determined using the lysimeter data.