Submitted to: International Workshop Characterization and Measurement Hydraulic Propertie
Publication Type: Proceedings
Publication Acceptance Date: November 21, 1997
Publication Date: N/A
Interpretive Summary: Properties of the soil surface directly influence how much water infiltrates during rainfall and irrigation, as well as, how much water is lost to evaporation from the soil surface. The amounts of infiltration and evaporation impact farmers' ability to produce crops economically, as well as, influencing the soil water balance and ultimately the movement of water rand any water borne contaminants that reach the water table. Over the lon term, tillage practices may influence these soil hydraulic properties, and some tillage practices may be more desirable from a soil water balance point of view. We used a combination of field and laboratory measurements to find the soil properties that resulted from 12 years of no-tillage (NT) and conventional tillage (CT) on two crop rotations: wheat-sorghum-fallow (WSF) and wheat-fallow (WF). Field measurements involved tests of a new multiple-reservoir tension infiltrometer. The new infiltrometer performed well and allowed field measurements not previously attainable. Measured water retention curves showed marked differences between NT and CT treatments. The more dense NT soils had a less steep drop off of water content as tension increased. The measured hydraulic conductivity (K) curves showed that K was greater in CT than in NT for both WF and WSF rotations over most of the water content range from 0.1 to 0.5 m**3 m**-3. This indicates that infiltration into conventionally tilled plots would be higher. The measured hydraulic properties are essential input data for computer models predicting crop water use, runoff, and soil water movement to the water table.
Technical Abstract: Soil surface hydraulic properties directly influence infiltration as well as the movement of soil water to the surface during drying. We measured surface soil hydraulic properties on 12-year-old no-till (NT) and conventional (stubble mulch) tillage (CT) plots; each of which was farmed with either a wheat-sorghum-fallow (WSF) or a wheat-fallow (WF) rotation. Single- and multiple-reservoir tension infiltrometers measured steady stat infiltration rates at four tensions (nominally 2.0-, 1.5-, 1.0-, and 0.5- kPa applied in that order). Initial soil water contents were measured by taking volumetric soil samples. Final soil water contents were inferred from the final measured tension at steady state and the soil water retention curve. Hanging water column and pressure plate techniques were used to measure the retention curves on undisturbed core samples. Wooding's equation was solved by nonlinear optimization for K(h) values. The K(h) and THETA(h) data were fitted to Mualem's and van Genuchten's forms for the retention and hydraulic conductivity curves, respectively, using the RETC program (r**2 >/= 0.99). The fitted water retention curves show marked differences between NT and CT treatments. The more dense NT soils had a less steep drop off of water content as tension increased. The fitted values of THETAs were close to the values of porosity from bulk density data except in the case of WSFCT which gave fitted THETAs of 0.52, somewhat below the porosity of 0.56. The overall fitted K(h) curves showed that K was greater in CT than in NT for both WF and WSF rotations over most of the water content range from 0.1 to 0.5 m**3/m**3.