Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2002
Publication Date: 2/1/2003
Citation: Colaizzi, P.D., Barnes, E.M., Clarke, T.R., Choi, C.Y., Waller, P.M. 2003. Estimating soil moisture under low frequency irrigation using the cwsi. Journal of Irrigation and Drainage Engineering. 129(1):27-35. Interpretive Summary: Existing crop water stress indices derived from remotely sensed data have demonstrated the potential to determine when to irrigate, but these indices do not directly provide an estimate of how much water to apply. In this study, a new approach was developed that combines the crop water stress index (based on remotely sensed crop canopy temperature) with more traditional evapotranspiration equations to infer soil moisture content in the root zone. The results of this study show promise to provide farmers and irrigation managers with an improved means to monitor crop water needs remotely. Additionally, the ability to better determine crop water status will promote agricultural water conservation.
Technical Abstract: The present study investigated the relationship between the Crop Water Stress Index (CWSI) and soil moisture for surface irrigated cotton (Gossypium hirsutum, Delta Pine 90b) at Maricopa, Arizona, during the 1998 season. The CWSI was linked to soil moisture through the water stress coefficient (Ks) that accounts for reduced crop evapotranspiration when there is a shortage of soil water. A stress recovery coefficient (Krec) was introduced to account for reduced crop evapotranspiration as the crop recovered from water stress after irrigation events. A Soil Water Stress Index (SWSI) was derived in terms of Ks and Krec. The SWSI compared reasonably well to the CWSI; atmospheric stability correction for the CWSI did not improve comparisons. When the CWSI was substituted into the SWSI formulation, it gave good prediction of soil moisture depletion (f DEP; when to irrigate) and depth of root zone depletion (Dr; how much to irrigate). Error was greatest for f DEP < 0.6 because cotton is less sensitive to water stress in this range.