278 JOHN ERNST WEAVER 



The atmometers at station No. 1 were on the south slope and 

 exposed fully to the heat of the sun and the prevailing south-west 

 winds. The graphs for these instruments have been plotted on 

 the chart in figure 2. The rainfall for Colfax is shown for periods 

 corresponding with those of the intervals between evaporation 

 readings, and each vertical space on the chart represents 1 cm. 

 of rainfall. It should be noted in this connection that the 

 average annual precipitation of 56 cm. occurs mostly during the 

 months when vegetation is dormant; the average annual rainfall 

 for June (a wet month in 1913) being only 2.54 cm. The graphs 

 indicate that the maximum evaporation occurred during the 

 latter part of July. At this time the water content of the soil 

 to a depth of 25 cm. had reached the non-available point^ and 

 the vegetation, including the deep crevice-rooted bunchgrass, 

 was rapidly drying up. The average daily evaporation for the 

 two instruments at station No. 1 during the 126 days of obser- 

 vation was 28 cc. The mean of the readings of these two instru- 

 ments is believed to express quite accurately the measure of the 

 evaporating power of the air and is therefore plotted and used 

 in comparison with similar graphs in figure 5, 



The prairie replaces the bunchgrass-rimrock type of vegetation 

 as one passes up over the sides of the canyons. The chief fac- 

 tors causing the transition are the increased depth and increased 

 water holding capacity of the soil. The first invaders are 

 Balsamorhiza sagittata, Lwpinus ornatus, Antennaria luzuloides, 

 Achillea millefolium lanulosa, Festuca ovina ingrata and Litho- 

 spermum ruderale Other prairie species appearing early in the 

 succession are Helianthella douglasii, Hoorebekia racemosa, Po- 

 tentilla hlaschkeana, Hieracium scouleri, Castilleja lutescens, 

 Koeleria cristata, Gaillardia aristata, Siversia ciliata, and species 

 of Aster; while Epilohium paniculatum, Clarkia pulchella, Poly- 

 gonum majus, Festuca pacifica, and Madia exigua — all char- 

 acteristic rimrock plants — tend to drop out. Agropyron under 

 the new conditions abandons its characteristic bunch habit, 

 in part, and breaks up into smaller clumps which may spread by 



' Wilting coefficient determinations were made by. Dr. L. J. Briggs, of the 

 Bureau of Plant Industry, to whom the writer is indebted. 



