Publication Date

5-1-2007

Degree Program

Department of Geography and Geology

Degree Type

Master of Science

Abstract

Soil moisture is a measure of the water content in a soil that is dependent upon precipitation, evapotranspiration, runoff, drainage, and irrigation. Nebraska is one of the few states that measures soil moisture in an extensive network that records weather variables on a daily basis. Daily soil moisture observations are collected from depths of 10, 25, 50, and 100 centimeters and analyzed in this research. Data from these stations are evaluated on the spatial and temporal scales using spatial interpolations, time-series analysis, and cross-correlations to better understand the variations of soil moisture in the Northern Great Plains. Spatial interpolation grids were created for May through October of 2005, after the data were grouped into datasets of weekly, biweekly, and monthly observations. These datasets were imported into a Geographic Information System (GIS) and the Ordinary Kriging method was applied for spatial interpolation. The interpolation parameters were set to create output surfaces of 4x4, 16x16, 32x32, and 64x64 kilometer grids for analysis of their variations. As expected, it is found that soil moisture content is higher in southeastern Nebraska and lower in the northwest. Changes in the grid size render small scale variations, however, the general pattern of estimated soil moisture distribution does not change. The temporal analysis concluded that the soil's physical properties have a much greater effect on soil moisture than a station's location within the east-west moisture gradient. Sandy soils were consistently drier, while silt and clay soils retained water for a longer duration. The topmost layer of soil experiences the greatest variation due to interactions with the surface boundary layer. The highest water content values for silty soils were observed in the summer months, while the highest values for sandy soils were observed in the spring and fall months. The most soil moisture variation occurs in the summer and fall, while the least amount of variation occurs in spring and winter. Cross-correlations, measuring the time-lag relationship, demonstrated the impact of soil physical properties on soil moisture by depth, the influence of precipitation on 10 and 25 cm depth soil moisture, and the impact of near-surface soil moisture on maximum surface temperature. A stronger relationship was observed between soil moisture and maximum surface temperatures than with precipitation and soil moisture at various depths.

Disciplines

Earth Sciences | Geography | Hydrology

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