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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #386529

Research Project: Precipitation and Irrigation Management to Optimize Profits from Crop Production

Location: Soil and Water Management Research

Title: Alfalfa ET and crop coefficients for the semi-arid U.S. Southern High Plains

item Evett, Steven - Steve
item Marek, Gary
item Colaizzi, Paul
item Brauer, David

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 7/25/2021
Publication Date: 11/8/2021
Citation: Evett, S.R., Marek, G.W., Colaizzi, P.D., Brauer, D.K. 2021. Alfalfa ET and crop coefficients for the semi-arid U.S. Southern High Plains [abstract]. 2021 ASA-CSSA-SSSA Annual International Meeting, November 7-10, 2021, in Salt Lake City, UT, Virtual. Paper No. 133029.

Interpretive Summary:

Technical Abstract: Alfalfa is one of two reference crops used as the basis for the Standardized Reference Evapotranspiration equations (ETsz) promulgated by ASCE in 2005 for use with growth stage dependent crop coefficients (Kc) to estimate crop ET (ETc = Kc ETsz). The other reference crop is grass. Because weather, pests, other environmental variables, and management can influence the value of ETsz on any given day, the ASCE 2005 standard proposed a set of equations to calculate reference ET for a tall (alfalfa) crop (ETrs) and a short (grass) crop (ETos). Among other parameters, the standard sets values for aerodynamic and canopy resistances, and for alfalfa it presumes a crop height of 0.5 m and leaf area index of 4.5. We grew alfalfa as a reference crop for four years under well-watered and fertilized conditions and showed that measured alfalfa ET closely matched the ASCE 2005 ETrs calculation when crop height was >= 0.5 m with full cover. We also showed that a two-surface (soil and canopy) solution of the energy and water balance equations, in which canopy temperature was implicit, closely matched ASCE 2005 ETrs. In the present work we expand the work on iterative solution of the implicit equations for calculation of alfalfa reference ET (ETri). We present daily crop coefficients for well-watered and fertilized alfalfa grown in our region showing a crop coefficient of ~1.1 for full cover alfalfa with height >= 0.5 m based on ASCE 2005 ETrs, and for comparison we present crop coefficients based on ETri.