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Abstract: Evaporation of citrus orchards has been widely studied, but differences in methodologies and management conditions have led to conflicting results, mainly due to differences in ground cover and soil evaporation. In this work the contribution of transpiration and soil evaporation has been studied in a drip-irrigated, clean cultivated mandarin (Citrus reticulata Blanco) orchard on a sandy soil in Southern Spain. Evapotranspiration (ET) was measured using eddy covariance while soil evaporation was determined with microlysimeters, during August 2000 and May 2001. Average ET was 2.6mmday−1 in August and 2.1mmday−1 in May. The crop coefficient (K c) was 0.44 and 0.43 in 2000 and 2001, respectively. The coefficient of transpiration (K p) was 0.30 in 2000 and 0.25 in 2001. The daily bulk canopy conductance (g c) ranged from 1.2 to 2.2 (average 1.8) mms−1 in 2000 and from 1.2 to 2.7 (average 1.9) mms−1 in 2001. A model of daily canopy conductance as a function of intercepted radiation was derived and applied to calculate the transpiration of orchards with different values of ground cover (GC). The ratio of transpiration over reference ET of mandarin orchards is linearly related to ground cover (K p =0.7GC). Calculated crop coefficients agree with values suggested by FAO for mature orchards (around 0.65) but are substantially lower than FAO values for young plantations. [Copyright &y& Elsevier]

Abstract: The reduction in agricultural water use in areas of scarce supplies can release significant amounts of water for other uses. As improvements in irrigation systems and management have been widely adopted by fruit tree growers already, there is a need to explore the potential for reducing irrigation requirements via deficit irrigation (DI). It is also important to quantify to what extent the reduction in applied water through DI is translated into net water savings via tree evapotranspiration (ET) reduction. An experiment was conducted in a commercial pistachio orchard in Madera, CA, where a regulated deficit irrigation (RDI) program was applied to a 32.3-ha block, while another block of the same size was fully irrigated (FI). Four trees were instrumented with six neutron probe access tubes each, in the two treatments and the soil water balance method was used to determine tree ET. Seasonal irrigation water in FI, applied through a full-coverage microsprinkler system, amounted to 842mm, while only 669mm were applied in RDI. Seasonal ET in FI was 1024mm, of which 308mm were computed as evaporation from soil (E s). In RDI, seasonal ET was reduced to 784mm with 288mm as Es. The reduction in applied water during the deficit period amounted to 147mm. The ET of RDI during the deficit period was also reduced relative to that of FI by 133mm, which represented 33% of the ET of FI during the deficit irrigation period. There was an additional ET reduction in RDI of about 100mm that occurred in the post-deficit period. [Copyright &y& Elsevier]

Despite the importance of garlic (Allium sativum L.) among vegetable crops, very little information is currently available on its crop water requirements which depend mainly on crop evapotranspiration (ET). The objective of this work was to measure ET of garlic crops and to develop a simple model for its calculation based on the crop coefficient approach. Measurements of ET were performed in two commercial crops of garlic grown in Co´rdoba (Spain) in 2000 and Ecija (Spain) during 2002. The crop coefficient reached a maximum between 1.2 and 1.3 from canopy closure to maximum leaf area index (LAI), and then declined to values around 0.6 at the end of the growing season. These changes were parallel to changes in surface conductance which showed a maximum around 35 mm/s and decreased to values below 10 mm/s at the end of the growing season, partly due to leaf aging and leaf senescence. The decrease in surface conductance was also related to changes in vapor pressure deficit (VPD), which suggests that the maximum Kc of garlic crops should decrease when VPD increases. Seasonal water-use efficiency was 2.8 g dry matter per kg ET which corresponded to around 5 g fresh product per kg ET. Sensitivity of garlic ET to changes in both aerodynamic and surface conductance were rather low which may allow the direct application of the Penman–Monteith equation to calculate crop ET. [Copyright &y& Elsevier]

Under localized irrigation, even when applying non-limiting amounts of water, there could be transpiration (T) limitations due to a limited wetted soil volume. To study under field conditions how drip-irrigated almond trees responded to a change in wetted soil volume, two treatments were established in summer 2012 in a drip irrigated almond orchard in Cordoba, Spain. One treatment (“ Large volume” ) was initially irrigated with micro-sprinklers (MS) to wet the entire ground surface, and then reverted to drip irrigation, while other was always kept under drip irrigation (“ Small volume” ). Continuous monitoring of T and measurements of soil moisture content, tree water status and trunk growth were carried out. Even though trees in both treatments were supplied with sufficient water, the MS application induced an increase in T and an improvement in water status in “Large volume” relative to “Small volume” . A reduction in the hydraulic resistance of the tree was also detected in “ Large volume” , as well as an enhancement in canopy conductance and tree growth. We concluded that there are situations in the field where almond tree transpiration is limited by an insufficient wetted soil volume, even when supplied with adequate water, due to a high hydraulic resistance during times of high evaporative demand. [ABSTRACT FROM AUTHOR]

Abstract: Crop coefficients (the ratio of actual and reference evapotranspiration, ET/ET o ) can be derived from vegetation indices (VIs) obtained by remote sensing. If ground meteorological stations are available to calculate ET o , then the FAO method for estimating the ET of actual crops may be applied in combination with series of satellite images if it is assumed that the crops are not water stressed. This approach was evaluated for two annual crops (cotton and garlic) and three tree crops (mandarin, olive, and peach) using measurements of evapotranspiration, made with the eddy covariance method, as ground truth. Thirty images acquired by the Landsat 5 TM and Landsat 7 ETM+ sensors were used to calculate VIs and derive crop coefficients. The assessment (based on 557 data pairs) led to an overall positive valuation of the method. The root-mean-square deviation over all estimates was 0.75mmday−1. It was concluded that the VI-ET o method is valid and robust for estimating spatially distributed evapotranspiration in large, irrigated areas. Weaknesses of the method were identified and new research to overcome these deficiencies is proposed. [Copyright &y& Elsevier]

Abstract: This study provides insight into the assessment of the spatio-temporal trends of chlorophyll fluorescence, narrow-band physiological indices, and structural indices acquired with a hyperspectral imager flown over a flux tower in a canopy characterized by small seasonal structural changes and a heterogeneous architecture. A total of seven flights between summer and autumn were conducted with a hyperspectral camera that captured 30cm resolution imagery and 260 spectral bands in the 400-900nm region. This enabled the identification of pure-vegetation tree crown pixels around an eddy covariance flux tower without shadow components or background effects. The hyperspectral imagery was used to study the temporal patterns of canopy fluorescence and reflectance indices related to physiology and canopy structure. The seasonal trends observed in the airborne indices and fluorescence and their relationship with gross primary production (GPP) demonstrated that vegetation indices mostly related to structure such as the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI) yielded non-significant relationships (r2 =0.17; p>0.05) due to the small structural changes in the canopy through the season. By contrast, physiological indices related to chlorophyll content (TCARI/OSAVI), light use efficiency (PRI570), and canopy chlorophyll fluorescence calculated through the Fraunhofer Line Depth principle (FLD3) showed a similar seasonal trend to that of GPP measured at the same time of the flights (r2 in the range 0.75–0.84; p<0.01). These results are consistent with the physiological trend observed during summer and autumn, which showed that chlorophyll content increased by 17.9% while the NDVI and the estimated tree crown projected LAI (LAIp) remained almost constant during the experiment (3% variation). The time-series hyperspectral dataset demonstrated that the seasonal trajectories of the NDVI and EVI were weakly related (p>0.05) to the physiological indicators such as TCARI/OSAVI, PRI570 and fluorescence. The spatial variability of the hyperspectral indices investigated through the coefficient of variation (CV) showed that fluorescence around the tower varied up to 17% at the time of the maximum stress (summer), while LAIp showed little variation during that time (CV=1.8%). After the summer stress period, the CV for fluorescence and chlorophyll content decreased in autumn down to 9%. This study demonstrates that small physiological changes occurring in an evergreen canopy were still captured by remote sensing physiological indices and high-resolution airborne fluorescence. These indicators are required for GPP monitoring when the vegetation dynamics are not captured by remote sensing structural indices. [Copyright &y& Elsevier]