Cotton (Gossypium hirsutum L.) is beginning to be produced on the Northern Texas High Plains as a lower water-requiring crop while producing an acceptable profit. Cotton is a warm season, perennial species produced like an annual yet it requires a delicate balance of water and water deficit controls to most effectively produce high yields in this thermally limited environment. This study measured the water use of cotton in near-fully irrigated, deficiently irrigated, and dryland regimes in a Northern Texas High Plains environment, which has a shortened cotton producing season, using precision weighing lysimeters in 2000 and 2001. The irrigated regimes were irrigated with a lateral-move sprinkler system. The water use data were used to develop crop coefficient data and compared with the F AO-56 method for estimating crop water use. Cotton yield, water use, and water use efficiency was found to be as good in this region as other more noted cotton regions. F AO-56 ET prediction procedures performed better for the more fully irrigated treatments in this environment.

Sprinkler irrigation efficiency declines when applied water intercepted by the crop foliage, or gross interception (Igross), as well as airborne droplets and ponded water at the soil surface evaporate before use by the crop. However, evaporation of applied water can also supply some of the atmospheric demands usually met by plant transpiration. Any suppression of crop transpiration from the irrigated area as compared to a non-irrigated area can be subtracted from Igross irrigation application losses for a reduced, or net, interception (Inet) loss. This study was conducted to determine the extent in which transpiration suppression due to microclimatic modification resulting from evaporation of plant-intercepted water and/or of applied water can reduce total sprinkler irrigation application losses of impact sprinkler and low energy precision application (LEPA) irrigation systems. Fully irrigated corn (Zea Mays L.) was grown on 0.75 m wide east-west rows in 1990 at Bushland, TX in two contiguous 5-ha fields, each containing a weighing lysimeter and micrometeorological instrumentation. Transpiration (Tr) was measured using heat balance sap flow gauges. During and following an impact sprinkler irrigation, within-canopy vapor pressure deficit and canopy temperature declined sharply due to canopyintercepted water and microclimatic modification from evaporation. For an average day time impact irrigation application of 21 ram, estimated average Igross loss was 10.7%, but the resulting suppression of measured Tr by 50% or more during the irrigation reduced Igross loss by 3.9%. On days of high solar radiation, continued transpiration suppression following the irrigation reduced Igross J. A. Tolk ([]) 9

Short‐season corn (Zea mays L.) hybrids may reduce irrigation requirements and permit earlier harvest. We measured and compared evapotranspiration (ET) of a short‐season (SS, Pioneer 3737) and full‐season (FS, Pioneer 3245) hybrid under full irrigation in 1994 at Bushland, TX, and examined differences in growth, yield, and water use efficiency (WUE). Both hybrids were planted the same day in contiguous 4‐ha fields (each field with a weighing lysimeter to measure ET directly), were irrigated simultaneously until the SS hybrid reached mid grain fill (R5 dent stage), and were managed for high productivity. Harvest was at the normal field‐dried grain water content of 136 g kg−1 for the region. Rainfall was 320 mm; 465 and 577 mm of irrigation was applied to the SS and FS hybrid fields, respectively. Seasonal ET was reduced with the SS hybrid (SS, 673 mm; FS, 802 mm), with the primary reduction after SS anthesis. Peak daily ET rates (some >10 mm d−1) were not affected by hybrid type. Grain yields (dry basis) declined from 1322 to 1130 g m−2, but grain water use efficiency (WUEg = grain yield/ET) was similar across hybrids: SS, 1.68 kg m−3; FS, 1.65 kg m−3. Dry matter (DM) was reduced by >390 g m−2 for the SS hybrid, but DM water use efficiency (WUEd = DM/ET) was identical for the two hybrids, at 3.02 kg m−3. The SS hybrid reached physiological maturity 12 d earlier than the FS hybrid and was harvested 11 d sooner. Leaf area index was >5.5 m−2 m−2 for the FS hybrid, but barely >4 m−2 m−2 for the SS hybrid. The WUEd, WUEg, and peak daily ET rates were not appreciably different for the two hybrids when fully irrigated, although seasonal ET was less with the SS hybrid. A shorter‐maturity hybrid can reduce ET and seasonal irrigation requirement, but it will not reduce the needed irrigation capacity (flow per unit area) by more than 5 to 10%, as that is largely dictated by the near‐maximum daily ET rate needed to avoid soil water deficits and a corresponding yield reduction. With prevailing regional pumping and production costs, the reduced production income with a SS hybrid would be more than six to eight times the saving in irrigation water cost, but this could be offset by higher grain marketing prices with an earlier harvest and by the opportunity for grazing income from a winter wheat (Triticum aestivum L.) double‐crop.

No abstract

Evapotranspiration (Et) information is needed for many applications in agricultural and natural resource management, but commonly used prediction equations have not been adequately tested far the dry, high radiation, windy conditions of the Southern Great Plains. In order to test such equations, two precise weighing lysimeters at Bushland, TX, were used to measure daily Et from wellwatered, irrigated grain sorghum [Sorghum bicolor L.) Moench] in 1987 and 1988. Forms of the Penman, Penman‐Monteith, Jensen‐Haise, and Priestley‐Taylor models were evaluated for prediction of potential evapotranspiration (Etp). The Penman‐Monteith model provided the best predictions of Etp and predicted well across the entire range of measured Et. Forms of the Penman model with empirically fit wind functions overpredicted Etp by 20 to 409/0, and aftempts to fit a linear wind function to our data were not satisfactory The Jensen‐Hake model, requiring only daily mean temperature and solar radiation data, overpredicted by 30%, but performed consistently across the range of Et measured. Modification of the Priestley‐Taylor model with vapor pressure deficit or temperature advection terms is recommended. Without such modification, the model overpredicts at low Et rates and underpredicts at high Et rates.