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Soil Moisture Studies

Shown during the installation of soil moisture sensors in an IMZ are from left: Glen Roebke, Todd Schimelfenig, and Ken Hubbard.  An angled drill guide is used to drill holes to the desired depth.  Todd is shown removing a gravimetric soil moisture sample prior to the placement of a soil moisture sensor.  Sensors are installed permanently at depths of 50 and 100 cm.  Shallow sensors (10 and 25 cm depths) are moved during field tillage operations.

Project Goal
The main goal is to obtain measurements of the soil water in the root zone. Soil water in the root zone is analogous to money in your checking account. There are deposits (effective rainfall and precipitation) and there are withdrawals (evapotranspiration and drainage). Soil water increases with irrigation or rainfall and decreases with drainage and evapotranspiration. Run-off from the soil surface reduces the effective precipitation or irrigation. Irrigation, rainfall, and soil water are measured in this project. The run-off and drainage are estimated using a physical model. Evapotranspiration measurements are made by the atmospheric flux group and estimates are available from empirical evapotranspiration equations.

Project Description
Research has documented a strong relationship between evapotranspiration during the growing season and the resulting productivity of a crop. This close tie between water loss and photosynthesis (carbon fixation) is due to the common pathway for both water loss to the atmosphere and CO2 uptake from the atmosphere. Both processes take place through the plantís stomatal openings which in turn are controlled by light, temperature, and available water. The water available to the plants varies with rainfall and irrigation and can be measured in the soil layer that contains the plantís roots. Thus, soil water or moisture can effect the degree of stomatal opening and the subsequent water loss/carbon fixation. It is this shared conduit of the water and carbon cycles that makes the soil moisture study critical to the overall project goals. The results of this project will be critical to choosing mitigation strategies that may be needed to reduce the negative impacts of global change due to increasing carbon dioxide in the atmosphere. Such strategies may be needed to ensure adequate supplies of food, water, and appropriate use of natural resources.

The group has collected continuous readings of soil water from the 10, 25, 50 and 100 cm depths in the soil. The group has also measured the precipitation and the rainfall that has fallen on the research sites by use of tipping bucket raingages. There are 3 IMZs with soil water measurements in Site 1 and Site 2 and 4 IMZs where these measurements are collected in Site 3. More information on monitoring soil water in Nebraska can be found at http://www.hprcc.unl.edu/soilm/home.html.

This graph shows the calibration curve for clay soils and the Theta Probe used in the study. The output from the sensor (in millivolts) is closely related to the volumetric water content of the soil.

Ken Hubbard

Todd Schimelfenig

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