SOIL MOISTURE AND 



EVAPOTEANSPIMTION 



Soil Moisture 



Soil moisture has been measured over a 

 period of 5 years at five sites and 6 years at an 

 additional site. These sites on the two study 

 watersheds represent four soil units (12, 16, 

 20, 21) and include measurements under 

 three vegetation types (mature aspen, aspen 

 sprouts, and grass-forb). Measurements were 

 made to a depth of 6 feet, using a neutron 

 moisture probe. 



Although late fall rains do contribute to 

 soil water recharge, most recharge occurs in 

 the late spring as a result of snowmelt. The 

 soil mantle is fully recharged at the end of the 

 snowmelt period which may occur anytime 

 between early May and the middle of June. 



There is evidence indicating that soil mois- 

 ture withdrawal begins early in the spring 

 when the snowpack may be several feet deep. 

 This withdrawal is difficult to assess since it is 

 masked by snowmelt recharge. However, it is 

 known that measurable depletion begins im- 

 mediately after snowmelt and continues to in- 

 crease at a rapid rate into July. Withdrawal 

 rates are greatest in the surface 3 feet, the 

 area of highest root concentrations. As the 

 growing season progresses and potential evap- 

 o transpiration increases, the actual evapo- 

 transpiration losses are limited by soil mois- 

 ture availability. As the surface soils dry, the 

 percent withdrawal from the deeper soil levels 

 increases and is restricted to removal by 

 deep-rooted brush and tree species. 



Summer rainfall is only a small portion of 

 the annual precipitation. These summer 

 storms seldom recharge more than the surface 

 few inches of soil, and this moisture is quickly 

 lost to evaporation or transpiration. 



Moisture depletion in the surface 6 feet of 

 soil is shown in figure 13 for several combina- 

 tions of vegetation and soils. The average 



5-year-maximum and 5-year-minimum mois- 

 ture contents are presented as the extremities 

 of the bar graphs. Moisture depletion under 

 the three aspen communities ranged from 6.9 

 to 8.5 inches, but depletion under the grass 

 and the aspen-sprout communities was only 

 3.9 and 4.6 inches, respectively, for the same 

 soil types. These relations suggest that a po- 

 tential water savings of 3 to 4 area-inches 



ASPENFORB 

 (SOIL UNIT 161 



ASPEN FORB 

 SOIL UNIT 211 



ASPEN FORB 

 SOIL UNIT 12: 



ASPEN 

 SPROUT FORB 

 SOIL UNIT 121 



GRASS 

 MEADOW 

 SOIL UNIT 161 



8.48 7 85 691 4 62 3.91 



AVERAGE MOISTURE DEPLETION (Inches) 



Figure 13.— A comparison of soil moisture 

 depletion in the surface 6 feet for several 

 soil and vegetation types. Average maxi- 

 mum and minimum moisture values are 

 represented by the top and bottom of the 

 bars. 



17 



