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<br /> <br />RAINFALL SIMULATION STUDY OF THE EFFECTS OF TRAMPLING ON RUNOFF AND WATER <br />QUALITY ON MANCOS SHALE RANGELAND4 <br /> <br />tv <br />W. <br />0)' Background <br />co <br /> <br />In the summer of 1981 a small drop-former type rainfall simulator (plot size <br />approximately 1 m2 (9 ft2)) was used to index the relative effects of <br />vegetative cover and livestock trampling on water, sediment, and salt yields <br />on a fine, gray, crusted Mancos shale-derived soil near Woodside, Utah (Fig. <br />7). The soil was described by Schafer (1981) as Soil A on a low-relief sha1,e <br />pediment surface and is described earlier in this report. A total of 360 <br />runoff events were simulated on 180 plots. The experimental design involved <br />three vegetative conditions, four. levels of livestock trampling, two ante- <br />cedent'moisture conditions and 15 replications. Data were also collected on <br />soil chemistry and bulk density, and runoff water chemistry. <br /> <br />The three vegetation conditions were: 1) mat saltbush present, 2) Nuttall's <br />saltbush present, and 3) bare. Trampling was achieved by leading a steer <br />across the study plots. The average intensities for minimal, moderate and <br />excessive trampling were 8, 35 and 60%. <br /> <br />Rainfall was applied approximately 5-9 cm/hr (2-3 1/2 in/hr) from 1.8 m (6 ft) <br />above the ground. Rain water was deionized prior to its application. Vo1u- <br />metri c samp1 i ng was used to determi ne di scharge. E1 ectri ca1 conducti vi ty (EC) <br />was measured throughout each run. Sediment was sampled from a composite of <br />the total runoff. Data were analyzed using analysis of variance (ANOVA) for a <br />complete multifactorial design (see Table :5). <br /> <br />Soil salinity at the stUdy site is described in Table 6. <br /> <br />Resu1 ts <br /> <br />The results of the ANOVA for trampling and cover ilre provid.::d ill T.\:,l<i 5. <br />Results are depict}dr_H1 i tdtively in Figures 8 n:l 1. 111 gellet."" iNcl'e~sed <br />level> of trdlllpl i ng increased the EC frOll1 331 tll 577 u.uhos/cm. dnd suspended <br />sediment from 7,197 to 9,324 ppm in the rUnoff. However, increased trampling <br />also increased infiltration and depression storage, resulting in decreased <br />volumes of runoff from the plots. The net result was a potential decrease in <br />total salt and sediment discharge with an increase in trampling level. <br /> <br />Vegetation also affected runoff and water quality from the plots. Increased <br />vegetation cover decreased runoff volumes and salt and suspended sediment con- <br />centrations. The vegetation-trampling interaction was insignificant ( =.1) <br />as it affected runoff volume and suspended sediment. It was significant <br />( =.003), however, as it affected salt concentrations. Vegetation cover <br />ranged from 0 to 34% on the experimental plots. The actual range of cover on <br />the SUbject rangeland site is only 5 to 10%. Realistically, this range of <br />vegetative cover cannot expected to greatly influence runoff, erosion or <br />salinity yields. <br /> <br />4 Detailed study results provided in USDI (1982b). <br /> <br />22 <br />