Using a subsurface drip irrigation system to measure crop water use

Abstract

Procedures are presented for determining crop water use and crop coefficients for a row crop, using a neutron scattering probe with an efficient subsurface drip irrigation system. One procedure is called the slope-projection method, and the other is called a covariance procedure. Field tests were conducted with full-season, narrow-row cotton (Gossypium hirsutum L.) on a well-drained, sandy soil in a semiarid environment over a 5-year period. The goal was to improve automated irrigation scheduling, by relating evapotranspiration (ET) to growing degree days (GDD). The result, using a Penman–Monteith reference ET, was an average midseason crop coefficient of 1.11, with a standard error of 0.056. With class A pan evaporation as the reference ET, the average midseason crop coefficient was 0.877, with a standard error of 0.029. A fifth-order polynomial for the pan-based crop coefficient as a function of GDD was programmed into a controller and used successfully to irrigate a field automatically for one season.