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United States Department of Agriculture

Agricultural Research Service

Title: Land Use Effects on Soil Hydraulic Properties

Authors
item Schwartz, Robert
item Unger, Paul
item Evett, Steven

Submitted to: International Soil Tillage Research Organization Proceedings
Publication Type: Proceedings
Publication Acceptance Date: July 2, 2000
Publication Date: N/A

Interpretive Summary: Tillage alters the soil properties that influence water movement. Likewise, reestablishment of grass on cropland will, over time, produce changes in soil properties that can influence the amount of plant available water. A study was conducted to characterize and compare soil hydraulic properties on adjacent native grassland, cropland, and Conservation Reserve Program (CRP) sites at three locations in the Texas High Plains. Steady state infiltration rates at a tension were measured to evaluate saturated and unsaturated water flow in soils. Soils were sampled to determine bulk density and water retention properties. Saturated hydraulic conductivities measured on sweep-tilled cropland soils were no different than saturated conductivities on grassland soils. However, average unsaturated conductivities of cropped soils were four times greater than grassland soils. CRP sites had the lowest saturated hydraulic conductivities, which suggests that, after 10 years, grasses had not fully improved changes in pore structure caused by tillage. Results of a 10-day simulation of evaporation from a bare soil suggest that the larger unsaturated conductivities measured for sweep-tilled cropland leads to increased evaporative losses of water as compared with grassland, CRP sites, and no-tillage cropland. These results suggest that tillage can reduce the amount of water available for plant growth and that precipitation use efficiency increases slowly over time after cropland is replanted to perennial grasses.

Technical Abstract: Tillage alters the pore structure and hydraulic properties of soils. Likewise, reestablishment of grass on cropland will, over time, produce changes in soil hydraulic properties that can influence the amount of plant available water. We conducted a study to characterize and compare soil hydraulic properties on adjacent native grassland, cropland, and Conservation Reserve Program (CRP) sites at three locations in the Texas High Plains. A tension infiltrometer was used to measure unconfined, unsaturated infiltration over a range of water tensions (0.05, 0.5, 1.0, and 1.5 kPa) at the soil surface. Intact soil cores were sampled within the Ap and Bt horizons to determine bulk density and water desorption curves, theta(h). Unsaturated hydraulic conductivity over the range of tensions K(h) was estimated using Wooding's equation for steady state flow from a disk source. The van Genuchten-Mualem model was simultaneously fitted to K(h) and theta(h) data to obtain parameter values for each land use treatment. Mean near-saturated hydraulic conductivities of cropland were not significantly different from grassland. However, at 1.5 kPa supply tension, cropped soils had a mean unsaturated conductivity four times greater than grassland. CRP sites had the lowest (P < 0.05) near-saturated hydraulic conductivities, which suggests that, after 10 years, grasses had not fully ameliorated changes in pore structure caused by tillage. Results of a 10-day simulation of evaporation from a bare soil suggest that the larger unsaturated conductivities measured for sweep-tilled cropland leads to increased evaporative losses of water as compared with other land uses.

Last Modified: 7/24/2014
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