The drop in soil moisture content usually began in late June at the 20-cm. depth, 

 and in early July at the 50- and 100-cm. depths. Moisture at a depth of 20 cm. usually 

 started to become limiting for plant growth in early July, but the time at which this 

 limiting point was reached varied by about 3 weeks over the 5-year period. Moisture 

 did not become limiting at the 50-cm. depth until mid- to late July or at the 100-cm. 

 depth until early August. The time that the limiting point was reached at these depths 

 varied over the study years by approximately 2 weeks. 



Recharge of moisture levels in the upper soil mantle began with the August and 

 September storms. The great variability in occurrence and intensity of these storms 

 between years results in an equally great variability of soil moisture recharge. Re- 

 charge at the 100-cm. depth did not ordinarily begin until October. By the end of 

 October the amount of recharge at all levels did not usually exceed that held at 15-atm. 

 tension . 



There were no consistent differences in moisture depletion at the 20-cm. depth on 

 different exposures. However, at depths of 50- and 100-cm., the moisture depletion 

 curves crossed below the 15-atm. level about a week earlier on southwest exposures than 

 they did on northeast exposures. At 7,100 feet, soil moisture became limiting at all 

 depths sooner than at 8,200 feet. The moisture curves for the lower stations crossed 

 below the 15-atm. level about 1 week earlier at the 20-cm. depth and about 2 weeks 

 earlier at the 50- and 100-cm. depths than those for the upper stations. These differ- 

 ences in time of soil moisture depletion were reflected in the vegetation. Plants 

 dried sooner on southerly exposures than on northerly and sooner at 7,100 feet than at 

 8,200 feet. 



Comparisons With Weather Bureau Stations 



U.S. Dep. of Commerce Weather Bureau records of temperature and precipitation 

 collected from the two nearest official stations were compared with those obtained in 

 this study from the station on the southwest exposure at 7,100 feet. The nearest 

 Weather Bureau station was 12 miles northwest in Virginia City, Montana, at an elevation 

 of 5,835 feet. The other Weather Bureau station, Ennis , Montana, was 14 miles north- 

 northeast at an elevation of 4,953 feet. Both of these stations are in mountain 

 valleys. Virginia City is in a rather narrow valley virtually enclosed by mountains. 

 Ennis is in the much broader Madison River Valley; the closest mountains are several 

 miles away. 



Maximum temperatures, both average and absolute, were approximately 8° F. higher 

 at Virginia City and 10° F. higher at Ennis than at the station at 7,100 feet (table 

 13). The maximum temperature difference between the station at 7,100 feet and that at 

 Virginia City represents a decrease of about 6.3° F. per 1,000-foot rise in elevation. 

 Comparison of records at the station at 7,100 feet with those at Ennis shows a decrease 

 of about 4.7° P. per 1,000-foot elevation rise. This is fairly comparable to the 5.4° 

 F. decrease per 1,000-foot elevation rise between the stations at 7,100 and 8,200 feet. 



Differences of minimum temperatures were much smaller than those of maximum temper- 

 atures (table 13). Minimums at Virginia City averaged about 3° F. warmer and at Ennis 

 about 2.5° F. warmer than those at the station at 7,100 feet. The respective decreases 

 amounted to 2.4° F. and 1.2° F. per 1,000-foot elevation rise, compared to a 2.5° F. 

 decrease per 1,000-foot rise between the grassland stations at 7,100 and 8,200 feet. 

 Topographic differences between Virginia City and Ennis probably account for the latter 

 having a slightly colder minimum in spite of being about 900 feet lower in elevation. 



Differences in precipitation between the station at 7,100 feet and those at 

 Virginia City and Ennis were not consistent with elevational differences (table 13). 

 Average monthly precipitation from May through October was 0.25 inch lower at Ennis 

 and 0.31 inch higher at Virginia City than that recorded at 7,100 feet. Both the 

 station at 7,100 feet and Ennis are on the east side of the Gravelly Range, whereas 

 Virginia City lies west of the divide. Thus, Virginia City is in a better position to 

 receive rains from summer storms that commonly approach from the west. 



23 



