Infiltration rate decreased significantly and sediment production increased significantly on a site with a silty clay surface soil devoid of vegetation following periodic trampling typical of intensive rotation grazing systems. The deleterious impact of livestock trampling generally increased as stocking rate increased. Damage was augmented when the soil was moist at the time of trampling. Thirty days of rest were insufficient to allow hydrologic recovery. Soil bulk density, aggregate stability, aggregate size distribution and surface microrelief were related to the soil hydrologic response of the trampling treatments. Many of the world's rangelands evolved in the presence and under the influence of grazing ungulates. However, the introduction and maintenance of domestic livestock on continuously or rotationally grazed pastures has the potential for altering botanical composition and cover (Ellison 1960) and soil physical properties (Klemmedson 1956, Reed and Peterson 1961). Modification of those parameters, either singly or in concert, may accelerate the natural erosion process and result in decreased on-site productivity, increased sediment production, and increased susceptibility of downstream flooding. As stocking rates of domestic livestock are increased under continuous year-long or season-long grazing, rainfall infiltration generally decreases while runoff and sediment loss increase (Alderfer and Robinson 1947, Rauzi and Hanson 1966, Rhoades et al. 1964). Heavy continuous grazing is generally detrimental to soil hydrologic characteristics, while the effects of moderate or light continuous grazing are significantly less deleterious and frequently not significantly different from each other (Blackburn 1984, Gifford and Hawkins 1978). Supporters of intensive rotation grazing (IRG) systems such as the short-duration grazing method propose that heavy stocking rates under some forms of rotational grazing may be advantageous Authors are presently environmental research scientist, USA-CERL, Environmental Divison,

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Infiltration rate and sediment production were assessed in oak, bunchgrass and sodgrass vegetation types in moderate continuous (MCG), heavy continuous (HCG), and intensive rotation (shortduration, SDG) grazing systems and in a livestock exclosure (LEX). Infiltration rate was related to the total organic cover and bulk density characteristics of the site (RJ = 36). The amount of cover was more important than type, indicating that protection of soil structure from direct raindrop impact was the primary function of cover on infiltration. The SDG and HCG pastures had lower total organic cover with correspondingly lower infihration rates compared to the MCG and LEX pastures. Bulk density, an indicator of soil structure, was significantly lower in oak mottes than in the grass interspace, but there was no significant difference between pastures. Sediment production was related to the total aboveground biomass and the bunchgrass cover of the site (P q .79). Obstruction to overland sediment transport and protection from the disaggregating effect of direct raindrop impact were the primary functions of the total aboveground biomass and bunchgrass cover. Total aboveground biomass was greatest in the oak motte and least in the sodgrass interspace, consequently the sodgrass interspace had the greatest amount of sediment production and the oak mottes had the least sediment production. Midgrass cover and total aboveground biomass in the MCG and LEX pastures was significantly greater than in the SDG and HCG pastures; thus sediment production from the MCG and LEX pastures was significantly lower than from the SDG and HCG pastures.

Summary1 Fragmentation of tiger bush landscapes in south-west Niger between 1960 and 1992 is evidenced by a reduction of percentage woody vegetation cover, changes in the spatial attributes of vegetation patches, and an increase in the spatial heterogeneity of the landscapes. The spatial patterns and dynamics of these landscapes were eectively captured using a combination of selected patch-based landscape metrics that measured speci®c aspects of the spatial pattern. 2 Derived from the spatial distribution of the alternating bands of vegetation and bare ground, lacunarity curves provide a particularly eective quantitative measure of the spatial pattern and dynamics of tiger bush landscapes in terms of percentage vegetation cover, spatial heterogeneity, and the domain of scale of the landscape. Lacunarity curves can be used to characterize landscapes in areas with dierent climates and topographic settings, and are an eective and parsimonious indicator of the fragmentation of tiger bush. 3 The dynamics of the vegetation bands during the fragmentation process was anisotropic. A signi®cantly larger proportion of woody vegetation reduction occurred in the downslope than upslope portions of woody patches, while the opposite was true for woody vegetation expansion. These results corroborate the hypothesis that tiger bush bands migrate upslope due to the upslope±downslope resource gradient across the vegetation band. 4 Fragmentation of the tiger bush landscapes reduced retention of water on site, signi®cantly increasing the landscape permeability to surface¯ow. When vegetation bands were well connected in 1960, no transects were found that allowed surface water percolation. That is, no path travelling through the bare ground areas was found to connect the upslope edge and downslope edge of any of the 200-m long transects, regardless of their width (50, 100 or 150 m). By 1992, within the same but now severely fragmented landscapes transects of all widths allowed water to percolate across them (44% if 50 m wide to 89% if 150 m wide). This increased landscape permeability to surface¯ow may have reduced the water available to the remaining fragmented vegetation bands and accelerated their degradation.

Rangelands have undergone-and continue to undergo-rapid change in response to changing land use and climate. A research priority in the emerging science of ecohydrology is an improved understanding of the implications of vegetation change for the water cycle. This paper describes some of the interactions between vegetation and water on rangelands and poses 3 questions that represent high-priority, emerging issues: 1) How do changes in woody plants affect water yield? 2) What are the ecohydrological consequences of invasion by exotic plants? 3) What ecohydrological feedbacks play a role in rangeland degradation processes? To effectively address these questions, we must expand our knowledge of hydrological connectivity and how it changes with scale, accurately identify ''hydrologically sensitive'' areas on the landscape, carry out detailed studies to learn where plants are accessing water, and investigate feedback loops between vegetation and the water cycle.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Allen Press and Society for Range Management are collaborating with JSTOR to digitize, preserve and extend access to Journal of Range Management. AbstractInterception, as a function of simulated rainfall intensity and duration, was determined for a midgrass [sideoats grama (Bouteloua curtipendula (Michx.) Torr.)] and a shortgrass [curleymesquite (Hilaria belangeri (Steud.) Nash)]. In addition, the redistribution of natural precipitation via plant interception was determined for live oak (Quercus virginiana Mill.) mottes. Interception storage capacity for sideoats grama and curleymesquite was 81 and 114% of dry weight, respectively. This difference was attributed to physical characteristics of the species and their respective growth forms. However, because sites dominated by sideoats grama had more standing biomass (3,640 kg ha-') than sites dominated by curleymesquite (1,490 kg ha-'), it was estimated that a sideoats grama dominated site had an interception storage capacity of 1.8 mm compared to curleymesquite dominated site with an interception storage capacity of 1.0 mm. Based upon precipitation event size and distribution for the study site at the Texas Agricultural Experiment Station near Sonora, Texas, the estimated interception loss for curleymesquite dominated sites was 10.8% of annual precipitation, compared to 18.1% interception loss for sideoats grama dominated sites. Only 54% of the annual precipitation reached mineral soil beneath the oak mottes as throughfall or stemflow. The remainder of the precipitation was intercepted by the motte canopy or litter layer and evaporated. Due to the water concentrating effect of stemflow, soil near the base of trees received about 222% of annual precipitation. Soil at a distance greater than approximately 100 mm from a tree trunk received only 50.6% of annual rainfall. Individual tree canopy width, height and depth measurements were insigniflcant predictors of stemflow and throughfall. Interception, throughfall and stemflow, expressed as percent of storm precipitation, were well-deflned curvilinear functions.Availability of water is one of the predominant factors influencing rangeland productivity. It has been demonstrated that plant interception can substantially influence the water budget of an area (Clark 1940, Kittredge 1948, Helvey and Patric 1965, Delfs 1967, Corbett and Crouse 1968, Douglass 1983, Hibbert 1983, and Seastedt 1985.To date, grass interception research has been characterized by a diversity of techniques that includes simulating rainfall on grass clippings arranged in a wire basket (Beard 1956), sealing the soil surface with Neoprene and measuring the amount of runoff (Corbett and Crouse 1968), measuring the amoun...

Drought is an ambiguous term, subject to expectation and the weight of emphasis on meteorological, agricultural, hydrological and socioeconomic dimensions. Uncertainty associated with the identification of drought often results in a lagged response in reducing stocking rates. This delay reduces vegetation cover, increasing the potential for accelerated erosion following the drought. The long-term consequences of accelerated erosion are a reduction of soil depth, a decline in soil structure and a decrease in infiltration rate and water storage capacity. Less water stored on a site hastens the onset of plant stress, effectively increasing the perceived frequency and consequences of drought. Management and policy tools must improve the integration of economic and ecological aspects of drought-induced de-stocking decisions, especially by incorporating the long-term irreversible costs of erosion.

Understanding tbe temporal response of infiltration rate and interrill erosion to selected livestock grazing strategies is necessary for the continued soil and water conservation of rangeland. Inflltration rate and btterrill erosion were sampled bimontbly from 1978-1984 on pastures grazed continuously (MCG) and moderately stocked (8.1 be AU-r); continuously (I-ICC) and heavily stocked (4.6 ha AU-r); bigb-intensity, low-frequency (IIILF) and moderately stocked (8-l; 12119 day, stocked at 8.1 ha AU-t); short duration (SDG) and heavily stocked (14-l; 4~50 day, stocked at 4.6 ha AU-r). The MCG and HILF pastures were able to recover from drougbts and maintain initial btflltration rates and btterrill erosion. In contrast, Miltration rates decreased and btterrlll erosion increased on HCG and heavily stocked SDG pastures. Tbe trend of infiltration rate and interrill erosion deterioration in tbe heavily stocked SDG and HCG pastures was not gradual; rather, it followed a stair-step pattern typified by decreasbtg condition durbrg drought and an inability to recover to pre-drougbt level during periods of above-normal precipitation. Tbe heavy stockbrg rate and climate rather than grazing strategy were the primary factors btiluencing tbe hydrologic responses. Infiltration rates were seasonally cyclic in the SDG end HCG pastures, but no significant seasonal trend could be identified in tbe MCG pasture. This was attributed to greater midgrass cover and litter accumulation in the MCG pasture which provided cover stability compared to less litter accumulation and a greater dombtance of seasonal sbortgrasses and forbs in the SDG and HCG pastures. Total organic cover was the most important factor determining infiltration rate. The midgrass bunch growth form and litter accumuiation were the most important factors brfbrencing interrill erosion. Both factors increased microrelief, and obstructed sediment transport and interrill erosion.