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United States Department of Agriculture

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Research Project: MANAGING LIMITED IRRIGATION AND RAINFALL FOR CROP PRODUCTION IN SEMI-ARID ENVIRONMENTS Title: Soil Water Content on Drip Irrigated Cotton: Comparison of Measured and Calculated Values Obtained with the Hydrus 2-D Model

Authors
 Bufon, Vinicius - Lascano, Robert Bednarz, Craig - Booker, Jill Gitz, Dennis

 Submitted to: Irrigation Science Publication Type: Peer Reviewed Journal Publication Acceptance Date: March 6, 2011 Publication Date: July 1, 2012 Citation: Bufon, V.B., Lascano, R.J., Bednarz, C., Booker, J.D., Gitz, D.C. 2012. Soil Water Content on Drip Irrigated Cotton: Comparison of Measured and Calculated Values Obtained with the Hydrus 2-D Model. Irrigation Science. 30(4):259-273. Interpretive Summary: As irrigation water becomes scarcer the method to supply a crop with its water requirement needs to be both economic and efficient. Such a scenario is taking place in the Texas High Plains, where water from the Ogallala aquifer continues to diminish and as a result farmers have adopted sub-surface irrigation. This method of irrigation consists of a buried drip tape to a depth of 12-16 inches on alternate 40-inch rows. This design was based on trial and error and economic considerations and not on specific soil properties and as a result many unknowns exist regarding drip positioning in the soil. One way to answer some of these questions is to use a computer model that calculates how water moves in a soil and the amount of water used by the crop. Such a model is called HYDRUS, which was developed by USDA-ARS scientists in California. This model is capable of calculating movement of water and heat in three dimensions, that is, as a function of soil depth (z), across a row (y) and in between rows (x), using sophisticated numerical schemes. This model was tested on a local soil, Amarillo series, and on a cotton crop. The test was to verify that the calculated values of soil water content matched the measurements and our results showed that the model can be used to calculate how water moves in the soil. The practical aspect of this finding is that HYDRUS model can be used as a management tool to design and tailor the installation of a drip tape for a specific soil type. For example, for a soil with high clay content, the model can be used to find out the ideal depth of installation of the drip tape, and emitter spacing and flow rate. The HYDRUS model was validated for the Amarillo soil and can be used to evaluate irrigation strategies. Technical Abstract: Crop irrigation with subsurface drip (SDI) is increasing in the semiarid Texas High Plains (THO). However, information on drip-tape positioning and irrigation strategies on the wetted soil area (WSA) is needed to optimize rainwater harvesting under well capacities of < 7 mm/d. Time and resources necessary to test SDI strategies for different conditions is sizeable to be determined by field experiments alone. However, a mechanistic model such as Hydrus-2D can quantify the effect of different installation geometrics and irrigation strategies. Our objective was to experimentally validate the Hydrus-2D in an Amarillo soil in THP to use the model in future evaluations of different irrigation frequency and timing strategies for SDI cotton. Results showed that Hydrus-2D calculated volumetric soil water content within +/- 3 percent of measured values, and simulation bias represented the smaller portion of the simulation error, indicating that the model can evaluate geometry and irrigation strategies.