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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 #328651

Title: Lysimetric evaluation of the APEX Model to simulate daily ET for irrigated crops in the Texas High Plains

item SALEH, ALI - Tarleton State University
item NIRAULA, REWATI - Tarleton State University
item Marek, Gary
item Gowda, Prasanna
item Brauer, David - Dave
item HOWELL, TERRY - Retired ARS Employee

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2017
Publication Date: 1/1/2018
Citation: Saleh, A., Niraula, R., Marek, G.W., Gowda, P.H., Brauer, D.K., Howell, T.A. 2018. Lysimetric evaluation of the APEX Model to simulate daily ET for irrigated crops in the Texas High Plains. Transactions of the ASABE. 61(1):65-74.

Interpretive Summary: Freshwater resources are becoming scarce and subject to nutrient pollution from agricultural use. Modeling tools such as the APEX model are used to evaluate the effects of management practices on water quality. Meaningful evaluation is dependent on accurate simulation of water balance components. However, limited work has been done to assess the model's ability to estimate crop water use (ET). In this study, we compared simulated ET from five methods to measured ET values from weighing lysimeters at USDA-ARS in Bushland, TX. Results indicated that APEX generally underestimated ET but performed adequately for simulating ET for crops grown in the semi-arid Texas High Plains.

Technical Abstract: The NTT (Nutrient Tracking Tool) was designed to provide an opportunity for all users, including producers, to simulate the complex models, such as APEX (Agricultural Policy Environmental eXtender) and associated required databases. The APEX model currently nested within NTT provides estimates of the changes in nitrogen (N), phosphorus (P), and sediment losses that are associated with alternative practices that are specified by the user. APEX is a field-level model that was developed in the early 1980s to assess the effects of management strategies on water quality. The accurate representation of water balance components is required for meaningful simulation of crop growth, biomass production, and subsequent sediment and nutrient transport. Evapotranspiration (ET) is oftentimes the largest component of the water balance, particularly in semi-arid regions. Several empirical equations have been developed to estimate ET. The following five methods, currently nested in APEX estimate ET for major cultivated crops: 1. Penman-Monteith (usually for windy conditions) – adds a logarithmic eddy diffusion function and canopy resistance to the Penman equations. 2. Penman – adds relative humidity, wind speed, and elevation of wind measurements to the required weather inputs. 3. Priestley-Taylor – requires radiation, as well as temperature e as an input. 4. Hargreaves – has two parameters which can be adjusted to calibrate the model for factors such as proximity to a major water body. 5. Baier-Robertson - is popular in Canada and is provided mainly for use in cold climates. This study was conducted to evaluate the accuracy of five ET equations currently available in APEX using both onsite measured climatic data and those from NTT standard databases. The average daily, monthly, and annual ET values predicted by each of the equations in APEX for a lysimeter field at the USDA-ARS CPRL at Bushland, TX was compared to those measured from 2001-2010 period. The Hargreaves and Samani and Baier-Robertson methods achieved the highest correlation with daily (R2 = 0.67 and Nash Sutcliffe Efficiency (NSE) = 0.63; R2 = 0.66 and Nash Sutcliffe Efficiency (NSE) = 0.64 respectfully) and monthly (R2 = 0.87 and Nash Sutcliffe Efficiency (NSE) = 0.86; R2 = 0.87 and Nash Sutcliffe Efficiency (NSE) = 0.84 respectfully) ET prediction as compared to measured climatic data. Similar results were obtained for ET using the standard climate data currently available in NTT. This is encouraging because of limited availability of measured climatic data in the majority of locations. The analysis of predicted versus measured ET values both during and outside of the growing season revealed better agreement during the former than the latter. The APEX model generally underestimated ET at both the daily and monthly levels but over predicted for cotton years possibly due to over prediction of leaf area index during the senescing stage. Overall, APEX was able to adequately predict daily and average monthly ET for major crops grown in the semi-arid Texas High Plains. These results should reinforce confidence in the APEX model's capacity to accurately simulate ET in fully irrigated farms.