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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]

Subsurface drip irrigation (SDI) has become an important irrigation application method in the U.S. Southern High Plains where pan evaporation exceeds 2,400 mm per year. Early research comparing SDI with spray sprinklers showed that SDI was over-applied when scheduling irrigations using crop coefficients developed using sprinkler irrigation. Thus, crop coefficients developed using SDI may be smaller than those developed using sprinkler irrigation. Grain corn was grown for two years on large, precision weighing lysimeters at Bushland, Texas, with two lysimeters irrigated by SDI and two by mid-elevation spray (MESA) irrigation. Data used in this study were for fields irrigated to replenish soil water in the top 1.5 m of the soil to field capacity, as indicated by weekly neutron probe readings (100% replenishment). Crop coefficients developed for SDI (Kc_SDI) were compared with those developed for MESA (Kc_MESA) using ASCE standardized reference ET equations. The value of Kc_SDI ranged from 0.83 to 0.89 of Kc_MESA for the two years. Values of Kc_SDI remained consistently less than Kc_MESA even after maximum leaf area index was reached, indicating that considerable evaporative loss from the soil surface occurred with MESA irrigation even after full canopy cover. When we shortened the initial period after planting from 30 to 20 d and followed FAO 56 recommendations for surface drip irrigation under full-cover plastic mulch, we calculated basal Kc (Kcb) values (ETo basis) that were reasonably close to our Kc values for SDI for the crop development and early mid-season periods but were greater than our data for the later mid-season and late season periods. [ABSTRACT FROM AUTHOR]

The NTT (Nutrient Tracking Tool) was designed to provide an opportunity for all users, including producers, to run complex simulation models, such as APEX (Agricultural Policy Environmental eXtender), with the associated required databases. The APEX model currently nested within NTT provides estimates of the changes in nitrogen (N), phosphorus (P), and sediment losses that are associated with management practices specified by the user. Five methods (Penman-Monteith, Penman, Priestley-Taylor, Hargreaves-Samani, and Baier-Robertson) for determining potential evapotranspiration (PET) are available as inputs for estimating actual ET. This study was conducted to evaluate the accuracy of the ET values obtained from the five PET equations currently available in APEX using both onsite measured climate data and data from the NTT standard databases. The mean daily, monthly, and annual ET values predicted by each of the equations in APEX for a lysimeter field at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas, was compared to values measured for the 2001-2010 period. APEX generally underestimated ET with all PET methods (mostly during growing seasons) at both the daily and monthly levels but overpredicted for years when cotton was grown as the major cash crop due to overprediction of leaf area index during the senescing stage for cotton. The underprediction of ET in growing seasons was possibly from underprediction of rainfall due to estimation of rainfall for missing data. Overall, APEX was able to adequately (R2 ≥ 0.82 and NSE ≥ 0.80) predict mean monthly ET for major crops grown in the semi-arid Texas High Plains region. These results should reinforce confidence in APEX's ability to simulate ET accurately for fully irrigated farms. ET predictions with the Hargreaves-Samani and Priestley-Taylor methods, which require limited data compared to the Penman and Penman-Monteith methods, were similar (p > 0.05, one-way ANOVA), with mean errors within 8.7% for measured weather data and 12.6% for NTT-generated weather data for both methods. This is encouraging because of the limited availability of measured climate data for the majority of locations in the world, including the U.S. [ABSTRACT FROM AUTHOR]