Environmental Engineering Reference
In-Depth Information
3.9 Subsurface Storage Above the Water Table
The storage term (
V
) in the water balance equation commonly represents the
amount of surface water in a wetland. For those wetlands with seasonal or ephem-
eral surface water, subsurface water storage also is an important hydrological
consideration. Moisture content of the exposed soil influences the transport of
oxygen and other gases, thereby affecting redox condition and biogeochemical
processes. Moisture conditions affect the viability of soil fauna and the growth of
plants adapted to high moisture environments.
After surface water in the wetland dries up, water loss from the wetland soil
continues, mainly due to transpiration by wetland vegetation, which causes the
water table to drop and the soil to become unsaturated. The volume of sediment
between land surface and the water table is called the vadose zone. The soil remains
nearly saturated immediately above the water table due to surface tension that holds
water in the soil pores. As soil dries and the water table continues to decline, it
becomes increasingly difficult for plant roots to extract water from the soil. As a
result, the rate of transpiration decreases, the rate of water-table decline decreases,
and the water table eventually reaches a relatively stable position. This condition
persists until something changes; most often the change is a subsequent recharge
event that adds water to the unsaturated sediments. The amount of water required to
saturate the soil completely and bring the water table to land surface is called the
moisture deficit. A wetland with a small moisture deficit can recover from a dry
condition relatively quickly when wet meteorological conditions return. Therefore,
subsurface moisture storage is an indicator of the resilience of a wetland to
fluctuations in water inputs.
Subsurface moisture storage is determined by the depth to the water table and
soil water content in the vadose zone. Methods for determining the position of the
water table are described in the section on groundwater flow. Here we describe
methods for measuring soil-water content and then introduce the concept of specific
yield,
S
y
, that relates subsurface storage to water-table depth.
Δ
3.9.1 Thermo-Gravimetric Method for Measuring
Soil Water Content
This method starts with collecting a sample of undisturbed soil in a metal cylinder
of precisely known volume (e.g., 100 cm
3
) using a soil corer, or inserting the
cylinder into the side wall of a soil pit. Care must be taken to fill the cylinder
completely while at the same time avoiding soil compaction. The top and bottom
of the sample are leveled using a metal scraper so that the soil volume is equal to
the volume of the cylinder, and sealed with plastic caps and electrical tape to
prevent evaporation. Samples are transferred to the laboratory and weighed using
a balance to determine the pre-drying weight. Samples are placed in an oven with
