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SPARSE CANOPY SYMPOSIUM INTRODUCTION

A General System to Measure and Calculate Daily Crop Water Use

Texas A&M Univ. Res. and Ext. Center, Rt. 3, Box 219, Lubbock, TX 79403-9757 and USDA-ARS, 3810 4th St., Lubbock, TX 79415 USA

r-lascano{at}tamu.edu

There is a need for an accurate method to calculate and to measure crop water use on real-time. We implemented a system that combines knowledge of crop water use and available technology to control the timely application of water. Our objective was to test the system and compare it to the empirical engineering approach that uses a crop coefficient to relate crop water use to a reference evapotranspiration. Technologies involved are the measurement of plant water use with stem flow gauges, of soil water with time domain reflectometry, and weather variables. Measurements are coupled with calculated values of crop water use obtained with the model ENWATBAL. A single computer controls all functions, for example, measurements, model execution, activation of water delivery system. The system was tested for a 2-yr period with cotton (Gossypium hirsutum L.) in Lubbock, TX, using surface drip irrigation. Field experiments were conducted on an Olton clay loam (fine, mixed, superactive, thermic Aridic Paleustolls). Comparison of measured and calculated values of crop transpiration and soil water evaporation were in close agreement. Simulated results indicated that for a 3-d frequency irrigation with small quantities of water the engineering approach lacks the resolution to accurately calculate daily requirements of cotton under the semiarid conditions of the Texas High Plains (THP). This is particularly true early in the growing season when predominant evaporative losses are from the soil and not from the crop. We conclude that the proposed system is general and can be applied to schedule irrigations based on accurate estimates of water loss.

Abbreviations: DOY, day of year • ENWATBAL, energy and water balance model • LAI, leaf area index (m² m^-2) • LEPA, low energy precision application • THP, Texas High Plains • TDR, time domain reflectometry • ET, evapotranspiration (mm) • ET_o, reference ET (mm) • ET_p, potential ET (mm) • E_c, crop transpiration (mm) • E_s, soil water evaporation (mm) • and, K_c, crop coefficient

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