Hydrologic interrelationships with vegetation and soil as affected by selected livestock grazing systems and climate on the Edwards Plateau

Abstract

Typescript (photocopy).The effects of grazing systems and climate on rangeland hydrology, soil and vegetation were assessed over a six year period at the Texas Agricultural Research Station, located 56 km southwest of Sonora on the Edwards Plateau, Texas. Bimonthly estimates of infiltration rate and sediment production in midgrass and shortgrass dominated interspaces were obtained using a mobile-drip-type rainfall simulator. Soil and vegetation parameters were also measured for each simulated rainfall plot. Permanent point-frame plots, vegetation line transects, pasture-wide soil samples and natural runoff were also used to verify the trends obtained on the simulated rainfall plots. Data were collected in pastures that were grazed under moderate continuous (MCG), heavy continuous (HCG), high-intensity low-frequency (HILF), and short duration (SDG) systems, and in a livestock exclosure (LEX). The pastures were stocked with a combination of cattle, sheep and goats at a ratio of 50:25:25, respectively. Total organic cover and clay content of the surface soil were the most influential factors of the infiltration rate model 2 (multiple regression model r^2 =.98). Cover breaks raindrop and thus protects surface soil structure. Clay content was associated with aspects of soil structure such as bulk density (r=-.77) and aggregate stability (r=.41). Sediment production estimation (multiple 2 regression model, r^2 = .96) was influenced most by midgrass cover and litter biomass. Bunchgrass and litter both acted as barriers that inhibited sediment transport. The MCG, HILF and LEX pastures demonstrated the ability to recover from droughts and maintain or improve infiltration rates, sediment production rates and midgrass relative dominance. Infiltration rates and midgrass dominance significantly decreased and sheet erosion significantly increased on the HCG and SDG pastures stocked 1.75 times the moderate rate. Changes in the HCG and SDG pastures were most pronounced during drought. The observed rate of recovery for the HCG and SDG systems was much slower than the moderately grazed MCG and HILF systems and the ungrazed LEX pasture. Infiltration rate was seasonally cyclic in the HCG and SDG pastures but was not in the MCG or LEX pastures