Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 1, 1999
Publication Date: N/A
Interpretive Summary: Efficient water management is rapidly becoming more important in the arid southwest as water costs and constraints on water availability increase. For efficient management of irrigation water for the short-season (early maturity) cotton produced in the region, better information is needed about the crop's water use requirements. Studies were conducted to determine the ewater use requirements for the short-season cotton. This information was then used to derive and test three crop water use curves to predict daily water use. The results indicated that the three curves closely predicted the actual daily cotton water use determined in the field. The crop curves can be used within a computerized irrigation scheduling program to assist farmers and consultants in achieving efficient water management for the short-season cotton.
Technical Abstract: Basal crop coefficients (Kcb) are used to predict crop evapotranspiration. Field studies were conducted in central Arizona to derive and test three time-based Kcb curves for an early maturity upland cotton. The cotton evapotranspiration (ETc) rates were determined from soil water depletion measurements made during small basin studies for the cotton grown under well-watered conditions in 1993 and 1994. Values for Kcb were developed with the ETc data and a grass-reference evapotranspiration (ETo). The resulting Kcb data were used to derive two Kcb polynomial curves indexed by days past planting (DPP) and cumulative growing degree days (CGDD). A third curve was derived from the data using the FAO-24 "straight-line" method. The curves were tested by comparing predicted ETc rates to soil water balance (SWB) ETc rates determined for the cotton in large basins. Weighted mean bias error (MBE) and mean absolute error (MAE) analyses indicated that tthe three curves predicted ETc for the large basins about equally well. Positive and negative biases in ETc rates tended to balance over the season, resulting in small values for MBE (-0.40 to 0.47 mm/d). Computed over all basins in a given year, the relative MBE (i.e., the MBE divided by the mean SWB ETc rate) for the curves varied between plus or minus 2%. The MAE (0.58 to 1.04 mm/d) and the relative MAE (8 to 14%) indicated that the accuracy of the predicted ETc was good for all three curves. When used in irrigation management procedures for the early-maturing cotton, any of the three Kcb curves presented should result in reliable estimates of ETc for climatic conditions and planting dates similar to those for these studies.