EFFECT OF ASSUMED UNIT GRADIENT DURING DRAINAGE ON THE DETERMINATION OF UNSATURATED HYDRAULIC CONDUCTIVITY AND INFILTRATION PARAMETERS1

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Abstract
The unit-gradient approximation, the determination of unsaturated hydraulic conductivities using this approximation, and the parameters of the Green-Ampt infiltration equation derived from those conductivities were evaluated in soils of four soil orders of the southern United States, encompassing a range of profile layering and complexity. The soils were Cecil (Typic Hapludults), Lakeland (Typic Quartzipsamments), Bethany (Pachic Argiustolls), and Konowa (Ultic Haplustalfs). The results indicate that the hydraulic gradients were greatly influenced by the nature and complexity of the soil profile and varied with time and depth. At some depths and times, the gradients were a factor of 2 to 5 smaller or greater than unity. A relatively large departure from the unit gradient occurred consistently in the top 30 to 40 cm of the soil. The hydraulic conductivity results showed that, from a practical point of view, appreciably discrepancies as a result of the unit-gradient approximation occurred in the upper soil horizons, at soil deppths less than 20 cm for a uniform profile to depths less than 65 for a more complex layered profile. For these depths, the hydraulic conductivities at low suctions, calculated using the unit-gradient assumption, were always smaller than the values measured gradients. Interestingly, the Green-Ampt wetting-front suction parameter derived from the unit-gradient hydraulic conductivities was generally close to the actual value for the upper soil depths, and its value for greater depths fluctuated around the actual value. Thus, the most serious effect of the unit-gradient assumption was the underestimation of the Green-Ampt parameter of saturated or nearly saturated hydraulic conductivity in the topsoil, which occurred in both uniform and layered soils. These results that tensiometers should be used to measure hydraulic gradients at least in the topsoil.