Advances in ET-based landscape irrigation management

While the use of evapotranspiration-based or ET-based scheduling to improve on the efficient use of water for irrigation has advanced considerably in recent decades, there is still a need to improve the estimation of ET in regions with multiple microclimates and where the vegetation is mixed or fetch is inadequate for measurement of ET using traditional methods. This paper describes the Landscape Irrigation Management Program (LIMP) model, which addresses these problems. More importantly, the paper describes methods to adjust reference evapotranspiration (ETo) for microclimate and to measure ET from small fields to provide “site-specific” coefficients for estimating ET. The LIMP model was developed as a scientific approach to estimating landscape water requirements, and the methodology is also helpful for estimating crop ET in regions with multiple microclimates and where crops differ in morphology, physiology, plant density, sensitivity to water stress, etc. A similar approach to LIMP was described in the Irrigation Association book “Irrigation Sixth Edition”. Although the LIMP model is useful, there is a paucity of literature on how to correct ETo for microclimate or how to determine the input coefficients for LIMP to estimate ET. In this paper, we discuss a method to estimate microclimate coefficients to account for spatial ETo variation and we discuss the use of the surface renewal method to measure in-situ ET, which can help to determine “site-specific” coefficients for locations with inadequate fetch to use other ET measurement methods. In addition to using the presented techniques for landscape ET estimation, the procedures are equally useful for estimation of ET in small cropped fields of uniform or mixed vegetation, riparian vegetation, climate controlled greenhouses, in undulating terrain, and regions with multiple microclimates.