Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
SIMULATION OF SOIL MOISTURE, EVAPOTRANSPIRATION AND DEEP PERCOLATION IN AGRICULTURAL WATERSHEDS
Abstract
Development and testing of mathematical relationships describing the infiltration, surface runoff, percolation and soil moisture status in the crop root zone in an irrigated or non-irrigated crop land are presented and discussed in detail. The developed relationships, along with Penman's evaporation formula, were then structured into a single digital computer model to simulate the hydrologic response of an agricultural watershed. This model can be used to determine the amount of precipitation and/or irrigation water which moves vertically downward and percolates out of the crop root zone on a daily basis for various soil conditions. Major daily climatic data and the soil moisture characteristics are required input data to the model. The model applies the developed equations to compute surface runoff, infiltration, potential and actual evapotranspiration, soil moisture storage and deep percolation from the crop root zone. Provision to estimate the crop irrigation water requirement is also included in the model. Infiltration and surface runoff are related to the antecendent moisture condition of the surface soil layer, soil cover, slope, and the amount of precipitation received in a specific time increment. Soil moisture movement in the crop root zone occurs under unsaturated condition and is determined by expansion of the one-dimensional differential form of the continuity equation. Potential evapotranspiration is based on Penman's equation and a set of monthly crop coefficients developed from experimental data. The actual crop evapotranspiration is then a function of its potential and is limited to the amount of available moisture in the crop root zone of the soil. Irrigation amounts during the crop growing season are computed based on the crop type, soil moisture deficiency and an empirically derived irrigation factor. Daily amount of percolation from the crop root zone is estimated using a statistical relationship developed by analysis of 7-year daily measured percolation and soil moisture data. This relationship has an exponential form and relates percolation to the degree of soil saturation, the saturation hydraulic conductivity and the slope of the soil moisture retention curve on log-log scale. To evaluate the performance of the complete model, it was applied to the 7-year continuous daily records from two weighing lysimeters. The results indicate excellent agreement between measured and simulated evapotranspiration, percolation and the soil moisture status in the root zone profile (r (GREATERTHEQ) .93). The developed equation for estimating surface runoff was tested separately. The major rainfall-runoff events and their corresponding soil moisture status, recorded in a small watershed during the 1964-1973 period, were used for this purpose. The results of the test showed an excellent relationship (r = .93) between computed and measured runoff for all events. Further, the model was combined with a groundwater flow model and applied to an agricultural watershed where both surface and groundwater factors are important and interdependent. The resultant hydrologic response including the groundwater levels are presented. A unique feature of model application was partitioning the watershed area into a large number of computational units, providing a grid work for data definition based on land capability classes and land use effects. An important step toward improving historical matching of groundwater levels was taken by this application.
Subject Area
Agricultural engineering
Recommended Citation
ARBAB, MAHMOOD, "SIMULATION OF SOIL MOISTURE, EVAPOTRANSPIRATION AND DEEP PERCOLATION IN AGRICULTURAL WATERSHEDS" (1980). ETD collection for University of Nebraska-Lincoln. AAI8111673.
https://digitalcommons.unl.edu/dissertations/AAI8111673