Apparent, or effective, infiltration rates on grassland hillslopes vary with rainfall intensity and flow depth because of the interaction between rainfall, runoff, and vegetated microtopography. The higher parts of the microtopography are occupied by greater densities of macropores and therefore have much greater hydraulic conductivities than the intervening microdepressions. On short hillslopes and plots the apparent infiltration rate is simply the spatial average of the saturated and unsaturated conductivities of this surface. The proportion of the surface which is saturated and the value to which the unsaturated conductivity is raised depends on the rainfall intensity. On longer hillslopes the downslope increase in flow depth in microtopographic depressions progressively inundates more permeable, vegetated mounds so that the hydraulic conductivity of a greater proportion of the surface is raised to its saturated value. For this reason the apparent infiltration rate increases downslope, even in the absence of spatial trends in any of the surface characteristics that affect infiltration. Apparent, or effective, infiltration rate depends on hillslope length. Consequently, steady state discharge does not increase linearly with distance downslope. These two fundamental relationships between infiltration, rainfall intensity, and runoff are analyzed on the basis of sprinkling-infiltrometer measurements and a mathematical model.

Effects of Rainfall, Vegetation, and Microtopography on Infiltration and Runoff

Relation of field studies and modeling in the prediction of storm runoff

Simulated rainfall was used to study infiltration rates and sediment production of28 plant communities and soils of five watershed areas in central and eastern Nevada. Two antecedent moisture conditions were used: soil initially air dry and initially at field capacity. InfiltratIOn rates and sediment production of the various soils are largely controlled by extent and surface morphology of dune interspace soils. Vesicular horizons are unstable in dune interspace surface soils. These horizons seldom occur in coppice dunes or in well-aggregated dune interspace soils. Infiltration rate is negatively related, and sediment production positively related to the occurrence and morphology of vesicular horizons. More sediment is produced from soils with antecedent moisture initially at field capacity than from'initially dry soil because of the instability of vesicular horizons when the soils were saturated. High-intensity summer thunderstorms in the Great Basin account for most of the runoff and sediment production from rangelands. Runoff is the major force initiating soil movement and transporting sediment. In order to manage rangeland watersheds effectively, factors influencing infiltration and sediment production need to be understood. Infiltration and erosion on rangelands have been studied for some time. Review of the literature includes Chapline [1929], Forsling [1932], U.S. Department of Agriculture [1940], Harper [1953], Gifford [1968], and Branson et al. [1972]. However, for Nevada rangeland there is practically no infor­ matIOn on infiltration and sediment production or on the in­ fluence of dune interspace areas and associated vesicular hori­ zons on infiltration and sediment production. The objective of this study was to determine which ground cover and soil pa­ rameters most influence infiltration and sediment production. METHODS

Factors influencing infiltration and sediment production of semiarid rangelands in Nevada

The hydrologic importance of grazing is receiving increased attention on rangelands in the United States. The literature on this topic is fragmented. This paper explores the available literature for information useful in understanding the hydrologic impacts of grazing intensity as related primarily to infiltration and runoff. Generally, data relative to range condition are not adequate for evaluating hydrologic impacts. Data relating grazing intensity to infiltration rates are available, yet distinct limitations are evident. These limitations are discussed in terms of identifying future research needs. The greatest need appears to be a detailed definition of the long-term effects of grazing (by year and season) on infiltration rates as a function of site, range condition, and grazing intensity. Once obtained, infiltration rates must be coupled with an appropriate method for generating runoff volumes, storm hydrographs, and long-term water yields.

http://srnr.arizona.edu/rnr/wsm462/Readings/gifford_hawkins_1978.pdf

Hydrologic impact of grazing on infiltration: A critical review

/pdf/forests-and-water-effects-of-forest-management-on-floods-3dq8vj1ut4.pdf

E. J. Dortignac

Papers

Infiltration Studies on Ponderosa Pine Ranges of Colorado