Soil Moisture Studies
||Shown during the installation of soil moisture sensors in an
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
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
||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