Date of Award:

5-2011

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Advisor/Chair:

Robert W. Hill

Abstract

Evapotranspiration (ET) of center pivot irrigated alfalfa was studied in the windy, arid, Curlew Valley, Northern Box Elder County, Utah, during the summers of 2009 and 2010. ET was estimated using eddy covariance (EC) and surface renewal (SR) techniques. ET estimates from the EC and SR analyses were compared with estimates using ASCE Standardized Reference ET Equation, with both dual and mean crop coefficients.

EC energy balance closure was 0.80, on average, in 2009 and 0.76 in 2010. The SR weighting parameter (α) was calculated through linear regression of EC and SR sensible heat flux estimates. Alpha was found to be 0.70 if EC energy balance closure was forced and 0.55 if closure was not forced. ET from SR analysis with α = 0.70 (ETSRα=0.70) was 409 mm in 2009 and 331 mm in 2010. ET from EC analysis with forced closure (ETECforced) was 390 mm in 2009 and 326 mm in 2010. In contrast, ETSRα=0.55 was 408 and 333 mm in 2009 and 2010, respectively, while ETECunforced was 315 and 251 mm in 2009 and 2010, respectively.

Combined ETECforced and ETSRforced were compared with estimated crop ET from the ASCE Std. Eq. with both dual and mean crop coefficients (ETcDual and ETcm, respectively). ETcDual was 689 mm in 2009, as compared to ETcm and ETEC-SRforced, which were 677 and 617 mm, respectively. In 2010 ETcDual was 674 mm, with ETcm and ETEC-SRforced being 629 and 576 mm, respectively. The Kcm approach more closely approximated the estimated wet soil evaporation determined from the ETEC-SRforced for the measurement conditions and stated assumptions.

ETEC-SR estimates were compared with irrigation application information to approximate field scale water balances. Effective precipitation plus net irrigation application (less wind drift and evaporation) were nearly equal to ETEC-SRforced for 2nd and 3rd crops of alfalfa in 2009 and 2010. No deep percolation was calculated using ETEC-SRforced; however, soil moisture measurements were not sufficient to verify that this was true. The water balances suggested that the fields were being underirrigated which may have caused salt accumulation in the soil, as evidenced by the low reported yields.

Comments

This work made publicly available electronically on May 11, 2011.

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