Karen S. Copeland

Evaluation of crop water stress index for LEPA irrigated corn

Abstract This study was designed to evaluate the crop water stress index (CWSI) for low-energy precision application (LEPA) irrigated corn (Zea mays L.) grown on slowly-permeable Pullman clay loam soil (fine, mixed, Torrertic Paleustoll) during the 1992 growing season at Bushland, Tex. The effects of six different irrigation levels (100%, 80%, 60%, 40%, 20%, and 0% replenishment of soil water depleted from the 1.5-m soil profile depth) on corn yields and the resulting CWSI were investigated. Irrigations were applied in 25 mm increments to maintain the soil water in the 100% treatment within 60–80% of the “plant extractable soil water” using LEPA technology, which wets alternate furrows only. The 1992 growing season was slightly wetter than normal. Thus, irrigation water use was less than normal, but the corn dry matter and grain yield were still significantly increased by irrigation. The yield, water use, and water use efficiency of fully irrigated corn were 1.246 kg/m2, 786 mm, and 1.34 kg/m3, respectively. CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures, and vapor pressure deficit values for the six irrigation levels. A “non-water-stressed baseline” equation for corn was developed using the diurnal infrared canopy temperature measurements as Tc–Ta = 1.06–2.56 VPD, where Tc was the canopy temperature (°C), Ta was the air temperature (°C) and VPD was the vapor pressure deficit (kPa). Trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Both the dry matter and grain yields decreased with increased soil water deficit. Minimal yield reductions were observed at a threshold CWSI value of 0.33 or less for corn. The CWSI was useful for evaluating crop water stress in corn and should be a valuable tool to assist irrigation decision making together with soil water measurements and/or evapotranspiration models.

Yield and Water Use Efficiency of Corn in Response to LEPA Irrigation

Center-pivot sprinklers are rapidly expanding on the Southern High Plains, and LEPA (low energy precision application) application methods are widely used in this region to reduce water application losses, to use the relatively low well yields, and to reduce energy requirements for pressurization. This study was conducted to evaluate LEPA irrigation response of corn (Zea mays L.) on slowly permeable Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll). The effects of irrigation amount were investigated in a field study during the 1992 and 1993 cropping seasons at Bushland, Texas. In 1992, a wetter than normal season, grain yields varied from 0.6 to 1.2 kg/m2 while in 1993, which was a season with slightly less than normal rain, grain yields varied from 0.4 to over 1.5 kg/m2 as irrigations increased from no-post plant irrigations to fully meeting the crop water use. Irrigation amounts for the full irrigation varied from only 279 mm for the wet year to over 640 mm for the more normal year. A significant relationship was found between grain yield and water use for the two years described as GY (kg/m2) = 0.00169 [WU (mm) – 147] with an r2 of 0.882 and a Sy/x of 0.10 kg/m2. Deficit irrigation of corn, even with LEPA, reduced yields by affecting both seed mass and kernels per ear. Generally, the grain yield was in proportion to dry matter yield. LEPA irrigation was shown to be efficient in terms of partitioning the applied water into crop water use. Irrigation amounts should not exceed 25 mm for alternate furrows (0.76-m rows) LEPA on the Pullman-type soils with furrow dike basins.

Sprinkler Irrigation Management for Corn — Southern Great Plains

ABSTRACT IN the Southern Great Plains, sprinkler-irrigated corn (Zea mays L.) yield is often limited by the irrigation water supply capacity. A field experiment conducted in 1987 determined sprinkler-irrigated corn responses which were used to modify and calibrate the CERES-Maize corn growth model. This validated model was then used to simulate corn water use and yield for various levels of irrigation water supply capacity. Effects of management decisions such as allowed soil water profile depletion and net irrigation application amount were studied. The yield, water use, and water use efficiency of fully-irrigated corn were 11.7 Mg/ha, 838 mm, and 1.40 kg/m^, respectively; and all decreased with irrigation deficits. The modified CERES-Maize model accurately simulated evapotranspiration, aerial dry matter yield and grain yield of the corn experiment. A 28-year simulation (1958-1985) at Bushland, TX, indicated that a net sprinkler irrigation supply capacity of 8 mm/d is necessary to avoid irrigation system related yield constraints on the slowly permeable Pullman clay loam soil. During those 28 years, a net irrigation supply capacity of 4 mm/d only reduced the mean yield by 12.5% from 11.15 Mg/ha to 9.75 Mg/ha but more than quadrupled the variance of the corn yield. With adequate net irrigation supply capacity, irrigation application amount (for 1- to 4-day irrigation frequencies) and allowed soil water depletion (up to 50% of the plant extractable soil water) did not greatly affect simulated crop yield but greatly affected the hydrologic efficiency for storage of precipitation and the net irrigation water requirement.

Radiometric surface temperature calibration effects on satellite based evapotranspiration estimation

Agriculture on the Texas High Plains (THP) uses approximately 89% of groundwater withdrawals from the Ogallala Aquifer, leading to steady decline in water table levels. Therefore, efficient water management is essential for sustaining agricultural production in the THP. Accurate evapotranspiration (ET) maps provide critical information on actual spatio‐temporal crop water use. METRIC (Mapping Evapotranspiration at High Resolution using Internalized Calibration) is a remote sensing based energy balance method that uses radiometric surface temperature (T s) for mapping ET. However, T s calibration effects on satellite based ET estimation are less known. Further, METRIC has never been applied for the advective conditions of the semi‐arid THP. In this study, METRIC was applied and predicted ET was compared with measured values from five monolithic weighing lysimeters at the USDA‐ARS Conservation and Production Research Laboratory in Bushland, Texas, USA. Three different levels of calibration were applied on a Landsat 5 Thematic Mappers thermal image acquired on 23 July 2006 to derive T s. Application of METRIC on a MODTRAN calibrated image improved the accuracy of distributed ET prediction. In addition, ET estimates were further improved when a THP‐specific model was used for estimating leaf area index. Results indicated that METRIC performed well with ET mean bias error±root mean square error of 0.4±0.7 mm d−1.

Surface Aerodynamic Temperature Modeling over Rainfed Cotton

Evapotranspiration (ET) or latent heat flux (LE) can be spatially estimated as an energy balance (EB) residual for land surfaces using remote sensing inputs. The EB equation requires the estimation of net radiation (Rn), soil heat flux (G), and sensible heat flux (H). Rn and G can be estimated with an acceptable accuracy. In computing H, radiometric surface temperature (Ts) is often used instead of surface aerodynamic temperature (To), as To is neither measured nor easily estimated. This may cause an underestimation of ET because H will be overestimated as Ts is typically larger than To for unstable atmospheric conditions. The objectives of this study were to (1) model To to improve the estimation of H and consequently ET for advective environments in the semi-arid Texas High Plains, and (2) assess the accuracy of the To model using three different methods (aerodynamic profile, lysimeter, and eddy covariance). A 6.5 m tower platform was used to measure profiles of wind speed, air temperature, and relative humidity in and above cotton canopy near a large weighing lysimeter managed under rainfed conditions at the USDA-ARS Conservation and Production Research Laboratory, Bushland, Texas. The To was modeled using H as a residual from the EB at the lysimeter location. Results indicated that To was better modeled as a linear function of Ts, air temperature, and surface aerodynamic resistance. Modeled To showed a very small estimation error (0.1% mean bias error and 3.8% root mean square error) when compared to To values measured using the aerodynamic profile data. Even though excellent results were found in this study, the model is only valid for dryland cotton with a leaf area index ranging from 0.2 to 1.3 m2 m-2. Furthermore, more research is needed to expand the To model to cover cotton grown under irrigated conditions and showing larger crop percent cover and leaf area index values, and under different environmental and atmospheric conditions.

COMPARING SEBAL ET WITH LYSIMETER DATA IN THE SEMI-ARID TEXAS HIGH PLAINS

SEBAL (Surface Energy Balance Algorithm for Land), a spatial evapotranspiration (ET) estimation method, has been applied with Landsat Thematic Mapper (TM) data throughout the world. However, it has never been tested for semiarid conditions of the Texas High Plains. In this study, SEBAL algorithm was applied to a Landsat TM image acquired on July 10, 2007 covering a major portion of the Texas High Plains. Performance of SEBAL was evaluated by comparing estimated ET with measured ET data on four large monolithic lysimeters at the USDA-ARS Conservation and Production Research Laboratory, Bushland, TX. Comparison of SEBAL-estimated instantaneous ET values with lysimetric measurements indicated that SEBAL may provide better ET estimates for irrigated fields. However, it performed poorly in predicting ET for fields under dryland management. This result may indicate that SEBAL might be sensitive to errors in the selection of the hot/dry pixel. Overall, SEBAL is a promising tool for mapping ET in the extensively irrigated Texas High Plains. However, more evaluation is needed for different agroclimatological conditions in the region.

Evapotranspiration and Crop Coefficients for Irrigated Sunflower in the Southern High Plains

Sunflower (Helianthus annuus L.) is a diverse crop grown for oil or confectionary uses in the Southern High Plains often under irrigation. Crop water use (evapotranspiration or ET) was measured in 2009 and 2011 in two 4-ha fields using two precision 9 m2 weighing lysimeters containing 2.3-m deep monoliths of Pullman clay loam soil. The fields were irrigated with a lateral move sprinkler system with nozzles ~1.7-1.8 m above the ground and ~1.5-m apart. The sunflower ET averaged 638 mm; seed yields averaged 308 g m-2; and the lysimeter water productivity averaged 0.49 kg m-3. The crop coefficients based on the FAO-56 curve method were 0.15 for Kcbini and 1.22 for Kcbmid based on the daily ASCE Reference ET (ETos). The Kcbmid based on the ASCE taller, rougher Reference ET (ETrs) was 0.80. Using a thermal-time base (growing degree day) for the crop coefficient did not improve the crop coefficient for the diverse planting dates in these seasons.

Estimating Seasonal ET from Multispectral Airborne Imagery: An Evaluation of Interpolation-Extrapolation Techniques

Irrigated agriculture in the Texas High Plains uses over 90% of the ground water from the Ogallala Aquifer. Efficient water use through improved irrigation scheduling is expected to moderate the aquifer decline rate and improve sustainability. Thus, an accurate estimation of spatial actual daily and seasonal evapotranspiration (ET) is needed. Remote sensing may be used for monitoring distributed actual ET. Therefore, during 2007, the Bushland Evapotranspiration and Agricultural Remote Sensing Experiment (BEAREX07) was conducted at the USDA-ARS Conservation and Production Research Laboratory (CPRL), Bushland, Texas. During BEAREX07, high resolution aircraft imagery (0.5 m pixel size in the visible and near-infrared bands and 1.8 m in the thermal band) were acquired during the cropping season using the Utah State University airborne multispectral digital system. Actual instantaneous ET was estimated using a two-source energy balance algorithm and extrapolation to daily values was performed using the evaporative fraction and the grass reference ET fraction (EToF). Cumulative/seasonal ET was estimated using EToF and cumulative grass reference ET. Data from four weighing lysimeters, in sorghum and corn fields, were used for evaluating ET predictions. Instantaneous ET was predicted with mean bias error (MBE) and root mean square error (RMSE) of 0.05 and 0.1 mm h-1 respectively. Daily ET was better extrapolated by the EToF method (error of 0.6±0.8 mm d-1, MBE±RMSE); while seasonal ET was slightly under-predicted for the short period of June and July by 8.9±30.4 mm. It appears that the aerodynamic resistance, in the soil sensible heat flux, has to be neglected under low biomass [leaf area index (LAI) ˜ 0.5 m2 m-2] and heterogeneous vegetation cover conditions. Furthermore, at LAI values around 5.0 m2 m-2, ET was over-predicted perhaps due to errors in estimating the fraction of LAI that is green, the clumping factor, the vegetation fraction, soil heat flux, and/or the soil resistance to heat flux term.

External Full-Time Vacuum Lysimeter Drainage System

Low-cost weighing lysimeters have been demonstrated with accuracies better than 0.1 mm. However, these low-cost lysimeters lack full-time vacuum drainage systems; and they lack access to the lysimeter tank for installation and maintenance of a vacuum system. Without frequent manual drainage, such lysimeters can become waterlogged. We designed, implemented, and characterized the performance of an automatic vacuum drainage system that can be added to a low-cost lysimeter externally (provided that drainage filters were installed in the lysimeter and plumbed to the outside). The system consists of a buried vertical cylindrical vacuum chamber, inside of which a drainage collection tank is suspended from a load cell. A small enclosure containing a vacuum pump, vacuum sensor, and ports for accessing the drainage chamber is situated above the vacuum chamber and level with the field surface. Disturbance of wind patterns and energy and water balances in the field is minimized by the buried system. At 0.0013 mm, accuracy of drainage measurement was nearly two orders of magnitude better than that of the lysimeter mass measurement, ensuring that the continuous drainage measurement may be included in the mass balance determination of evapotranspiration (ET) without diminishing the accuracy of ET values. The system design, installation, and testing are described.

Forage Sorghum Evapotranspiration and Crop Coefficients

Forage sorghum, especially brown mid rib (BMR) hybrids, offers attractive silage options in place of corn and has excellent ethanol conversions compared with other forage sorghums. The purpose of the study was to measure the evapotranspiration (ET), yield, forage quality, and water use efficiency of a fully irrigated BMR forage sorghum hybrid grown for silage in a semi-arid, advective environment. One monolithic weighing lysimeter (3 m x 3 m by 2.3 m deep) was used in 2006 and 2007 to measure the ET of forage sorghum in a 4.2 ha field. The crop coefficients (Kc) were computed using both the ASCE standardized short- and tall-crop reference ET equations (ETo and ETr, respectively) and contrasted with Kc values for grain sorghum and corn for grain in this environment. The discussion will focus on the crop water use, forage quality, and water use efficiency aspects of the study and water conservation of forage sorghum compared with irrigated corn in this region.