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Corn (Zea mays L.) is a major irrigated crop grown in the Southern High Plains including the Texas Panhandle. Irrigation from the Ogallala Aquifer is required to sustain profitable corn production in the region by supplementing inadequate and erratic rainfall. Effective irrigation scheduling works to extend limited groundwater resources by avoiding water losses associated with runoff and/or percolation below the root zone. The use of crop coefficient (Kc) and reference evapotranspiration (ETo) values to estimate daily crop water use (ETc) remains an effective scheduling tool that can complement other irrigation scheduling approaches. Both Food and Agriculture Organization (FAO-56) piecewise and curvilinear Kc values for corn are found in the literature. However, advances in corn genetics have led to questions about the applicability of Kc values developed using legacy corn hybrids to irrigation of modern drought-tolerant (DT) hybrids. Lysimeter-derived Kc values for legacy corn hybrids grown in large weighing lysimeter fields at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas, were compared with those derived from a modern DT corn hybrid recently grown in the same fields. Results indicated that although midseason daily Kc values were similar for all hybrids, average season length was ~25 days shorter for the modern DT hybrid, characterized by a shortened initial growth period followed by more rapid increase of Kc during the development period. However, plots of Kc over thermal time illustrated that the differences in season length were likely attributable to later planting dates associated with the DT corn hybrids. Average seasonal water use was 730 and 811 mm for the legacy and modern DT hybrids, respectively (three years each), with corresponding average yields of 1.2 and 1.4 kg ha-1. Results suggest that published Kc and Kcb values developed with legacy corn hybrids remain largely applicable to modern DT corn hybrids when used with accurate estimates of effective canopy-based growth stages and climate-specific Kc functions. [ABSTRACT FROM AUTHOR]

Water scarcity due to drought and groundwater depletion has led to increased interest in deficit irrigation strategies that reduce irrigation requirements while maintaining profitable yields. This has resulted in an increase in the number of modeling studies aimed at evaluating crop response to limited irrigation strategies. However, the ability of widely used crop simulation models to accurately represent responses to limited irrigation has not been thoroughly evaluated. The primary objective of this study was to determine the efficacy of DSSAT-CERES-Maize (ver. 4.6.1.0) to simulate leaf area index (LAI), crop evapotranspiration (ET), and yield response to full (100%) and limited (75% and 50%) irrigation regimes for two corn varieties. Comparisons of simulated and measured data from full and limited irrigation treatments of two drought-tolerant corn hybrids (DuPont Pioneer AQUAmax P1151HR and Pioneer 33D49) grown in the Texas Panhandle in 2013 and 2014 were evaluated. Simulated in-season daily crop ET values for P1151HR grown in 2013 were also compared to those measured by precision large weighing lysimeters at Bushland, Texas. Additionally, a comparison of simulated and measured soil water content (SWC) within the root zone was performed for P1151HR grown in 2013. Simulated LAI for fully irrigated treatments approximated measured values reasonably well, although manipulation of plant genetic parameters failed to match measured LAI during the period between maximum LAI and the beginning of crop senescence in the 50% irrigation treatments. Similarly, simulated yield values approximated measured values for the fully irrigated treatments, while considerable overestimation of yield occurred in the limited irrigation treatments for both varieties. However, consistent overestimation of both LAI and yield for the limited irrigation treatments suggests a functional relationship between LAI and yield. Further, DSSAT overestimated crop ET by 16% for fully irrigated P1151HR and by 40% for limited irrigation treatments in 2013 as compared to measured lysimeter values. Corresponding underestimations of SWC were also observed in neutron probe measurements for both treatments. Overestimation of ET and yield and corresponding underestimation of SWC in limited irrigation treatments were mainly due to overestimation of LAI in those treatments, indicating a potential deficiency in the water stress algorithms. Additional comparisons of agronomic and lysimeter-based water balance data are needed to corroborate the findings of this study. Further investigations into the calculation of reference evapotranspiration (ETo), crop coefficients, and water stress functions in DSSAT are needed in order to provide suggestions for model improvement. [ABSTRACT FROM AUTHOR]

Drought-tolerant ( DT) maize ( Zea mays L.) hybrids have potential to increase yield under drought conditions. However, little information is known about the physiological determinations of yield in DT hybrids. Our objective was to assess radiation-use efficiency ( RUE), biomass production, and yield in two hybrids differing in drought tolerance. Field experiments were conducted in 2013 and 2014 with two hybrids, P1151 HR ( DT hybrid) and 33D49 (conventional hybrid) under well-watered (I100) and drought (I50) conditions. I100 and I50 refer to 100 % and 50 % evapotranspiration requirement, respectively. On average, P1151 HR yielded 11-27 % greater than 33D49 at I100 and about 40 % greater at I50, At I100, greater yield in P1151 HR was due to greater biomass at physiological maturity ( BMpm) resulting from greater post-silking biomass accumulation ( BMpost). At I50, both hybrids had similar BMpm but P1151 HR showed a higher harvest index and greater BMpost. RUE differed significantly (P < 0.05) between the hybrids at I100, but not at I50. At I100, the RUE values for P1151 HR and 33D49 were 4.87 and 4.28 g MJ−1 in 2013, and 3.71 and 3.48 g MJ−1 in 2014. At I50, the mean RUE was 3.89 g MJ−1 in 2013 and 3.16 g MJ−1 in 2014. Results indicate that BMpost is important for maintaining high yield in DT maize. [ABSTRACT FROM AUTHOR]

The operational Simplified Surface Energy Balance (SSEBop) approach was applied on 14 Landsat 5 thermal infrared images for mapping daily actual evapotranspiration (ETa) fluxes during the spring and summer seasons (March-October) in 2006 and 2007. Data from four large lysimeters, managed by the USDA-ARS Conservation and Production Research Laboratory were used for evaluating the SSEBop estimated ETa. Lysimeter fields are arranged in a 2x2 block pattern with two fields each managed under irrigated and dryland cropping systems. The modeled and observed daily ETa values were grouped as "irrigated" and "dryland" at four different aggregation periods (1-day, 2-day, 3 day and "seasonal") for evaluation. There was a strong linear relationship between observed and modeled ETa with R2 values ranging from 0.87 to 0.97. The root mean square error (RMSE), as percent of their respective mean values, were reduced progressively with 28, 24, 16 and 12% at 1-day, 2-day, 3-day, and seasonal aggregation periods, respectively. With a further correction of the underestimation bias (-11 %), the seasonal RMSE reduced from 12 to 6 %. The random error contribution to the total error was reduced from 86 to 20% while the bias' contribution increased from 14 to 80% when aggregated from daily to seasonal scale, respectively. This study shows the reliable performance of the SSEBop approach on the Landsat data stream with a transferable approach for use with the recently launched LDCM (Landsat Data Continuity Mission) Thermal InfraRed Sensor (TIRS) data. Thus, SSEBop can produce quick, reliable and useful ET estimations at various time scales with higher seasonal accuracy for use in regional water management decisions. [ABSTRACT FROM AUTHOR]

Expansion of cotton (Gossypium hirsutum L.) production in the Ogallala Aquifer region of the United States can be attributed to early maturing cultivars, rising energy costs, and declining groundwater levels. The feasibility of growing cotton relative to the availability of heat units (HU) in this region has not been determined. In this study, a 30-yr (1971-2000), county-wide, daily maximum and minimum air temperatures database was developed to assess HU availability in the region. The time of planting used to initiate HU accumulation during the growing season was based on the estimated daily minimum soil temperature at planting depth. Linear regression models to estimate daily minimum soil temperature at planting depth using air temperature were developed for each of the climatic regions in the study area. The growing season was terminated with the first freeze or 15 October, whichever occurred first, and this was considered the harvest date. Total heat units (THU) based on the long-term annual averages and exceedance probability of 0.99 (every year) and 0.75 (3 out of 4 yr) were estimated and used to identify counties that are suitable for cotton production. Of the 131 counties evaluated, 110 received 1000 (°C) or more HU in 3 out of 4 yr. Based on heat unit availability, cotton is a suitable alternative crop for all counties in the Texas and Oklahoma panhandles and for the majority of counties in southwestern Kansas. Management uncertainties, such as irrigation efficiencies, soil types, fertilizer and pest management practices, and planting and harvest schedules, may require further consideration to determine the feasibility of cotton production in the region. [ABSTRACT FROM AUTHOR]