USGS Home
Contact USGS
Search USGS

U.S. GEOLOGICAL SURVEY
Data Series 284

Back to Table of Contents

Selected Soil-Moisture Data At Depth

At depth, soil-water content under natural-site and simu­lated waste-site conditions was monitored at four experimental sites: one vegetated, native soil profile; one devegetated, native soil profile; and two non-vege­tated, simulated waste trenches with disturbed soil used as backfill (figs. 2 and 3). The simulated waste trenches have 208-L soil-filled drums buried at depths from 1.5 to 2.5 m and 3.5 to 4.5 m; soil-water content is not determined for these depths because of the influ­ence of simulated waste on neutron-probe readings. Soil-water content is reported in volumetric units of cubic meter per cubic meter (volumetric water content).

Complete soil-water content data and the associated calibration equations are available in appendixes G–H. Volumetric water-content data by date and depth for each of the four experimental sites are listed in appendix G, and volumetric water-content data by date and depth for each individual neutron-probe access tube located at the experimental sites are listed in appendix H.

Volumetric Water-Content Profiles

Variations in volumetric water content with depth were measured periodically at ADRS to define moisture levels at the different depths within the sedimentary geology and to identify temporal changes in the profile. The deepest access tube (VS2, fig. 2) at ADRS measures a vegetated native-soil profile to a depth of 29.75 m. Figure 17 shows selected water content profiles obtained during the 5-year data collection period at the vegetated, native soil profile. Within the upper 1 m of the soil profile, water content ranged from a low of about 0.02 m³/m³ to a high of about 0.12 m³/m³ as shown by profiles of August 4, 2004, and January 25, 2005, respectively. For the 5-year data set (appendix G), the average volumetric water content within the upper 1 m of soil at the vegetative native-soil profile averaged about 0.05 m³/m³. The devegetated native-soil profile and the two non-vege­tated simulated waste trench sites averaged about 0.07 m³/m³ within the upper 1 m of soil. The difference in average volumetric water content between the vegetated site and the three sites without vegetation is attributed primarily to loss of root-zone soil-moisture by plant transpiration. The highest water content in the vegetated native-soil profile averaged about 0.14 m³/m³ at a depth of 25.25 m. At depth, moisture retention appears to be primarily a function of grain size within the varying basin fill. Clay lenses in sediments tend to have higher water content, whereas gravel layers have lower water content.

Figure 17 shows that temporal changes in soil moisture with depth are limited to the upper profile, generally in the upper 1 m, whereas below several meters the variation with depth reflects changes in lithology that does not appreciably change in time. Below several meters the profiles in figure 17 show profile changes of less than 0.02 m³/m³ at any given depth, and these changes are within the measurement error estimated at 0.017 m³/m³ based on calibration uncertainties (Andraski, 1997). In the upper 1 m of the soil profile, substantial moisture is derived from local rain events. Variations in soil moisture in the upper 1 m can be accounted for by the downward redistribution of water from precipitation, which infiltrates the soils after a storm event and by increased soil-water discharge by ET.

Figure 18 shows in more detail the temporal variations in soil moisture within the upper 5.5 m for each of the four experimental sites from May 2004 through August 2005. Figure 19 compares the water content between the four experimental sites at selected depths over time (2001–05). Significant rain events occurred during the 5 months from October 2004 through February 2005 that totaled about 217 mm of precipitation (fig. 13). These rain events are first indicated by the October 2004 profile in figure 18. All four experimental sites show measurable temporal variations in soil-water content in response to precipitation recharge in the upper 1.25 m of the soil profile. The August 2005 profile indicates that water from winter precipitation reached a depth of 2.75 m at the devegetated, native soil site (fig. 18B), although for the same profile at the vegetated native-soil site (fig. 18A), winter precipitation penetrated the soil to a depth of 1.75 m. Below these depths, downward movement of water from precipitation cannot be substantiated given that profile differences are equal to or less than the inherent uncertainty of the soil moisture measurements. The change in water content profiles from August 2004 to August 2005 at the devegetated native-soil site (fig. 18B) graphically illustrates the maximum recorded downward redistribution of soil moisture at the four experimental sites. This is the deepest documented temporal variation in soil-water content at the ADRS since soil moisture research began in 1983.

Back to Table of Contents