Evapotranspiration Summary

Derived from HSPEXP

In most hydrologic regimes, the volume of water that leaves a watershed as evapotranspiration (ET) exceeds the total volume of stream flow, making it an important aspect of the water budget. There are two separate issues involved in estimating evapotranspiration:

1. Potential ET (PET): ET potential or demand is supplied as an input time series, typically using U.S. Weather Bureau Class A pan records plus an adjustment factor. The data are further adjusted for cover in the parent subroutine PWATER. The PET is used to calculate actual ET.
2. Actual ET (TAET): TAET is usually calculated as a function of moisture storages and the potential. It is estimated by trying to meet the demand from five sources, in the order listed below. The sum of the ET from these five sources is the TAET from the land segment. Additionally, ET data is adjusted when snow is considered. See the Technical Note concerning SNOW for more information.

The first source from which ET can be taken is the active groundwater outflow or base flow. This simulates the effects such as ET from riparian vegetation in which groundwater is withdrawn as it enters the stream. The user may specify the parameter BASETP the fraction, if any, of the PET that can be sought from base flow. That portion can only be fulfilled if outflow exists.

Any remaining potential not met by actual base flow evaporation will try to be satisfied by ET from interception storage. Unlike base flow, there is no parameter regulating the rate of ET from interception storage. The demand will draw upon all of the interception storage unless the demand is less than the storage. The parameter for interception storage (CEPSC) can vary monthly to account for seasonal land use practices. When the demand is greater than the storage, the remaining demand will try to be satisfied through ET from upper zone storage (UZS).

There are no special ET parameters for the upper zone, but rather ET is based on the moisture in storage in relation to its nominal capacity. Actual ET will occur from the UZS at the remaining potential demand if the ratio of UZS/UZSN, upper zone storage to nominal capacity, is greater than 2.0. Otherwise, the remaining PET demand on the UZS is reduced; the adjusted value depends on UZS/UZSN.

Any remaining demand is attempted to next be satisfied by ET from active groundwater. AGWETP is the fraction of the remaining PET than can be drawn from areas having vegetation drawing directly from groundwater, such as marshes or wetlands. This portion of the ET demand can be met only if there is enough active groundwater storage to satisfy it. Any remaining potential will try to be met by lower zone storage.

The lower zone storage (LZS) is the last storage from which ET is drawn. ET from the lower zone is more involved than that from the other storages. ET from the lower zone depends upon vegetation transpiration. ET opportunity will vary with the vegetation type, the depth of rooting, the density of the vegetation cover, and the stage of plant growth along with the moisture characteristics of the soil zone. These influences on the ET opportunity are lumped into the LZETP parameter. Unlike the other ET parameters, LZETP can be input on a monthly basis to account for temporal changes in the above characteristics. If the LZETP parameter is at its maximum value of one, representing near complete areal coverage of deep rooted vegetation, then the potential ET for the lower zone is equal to the demand that remains. However, this is normally not the case. Usually vegetation type and/or rooting depths will vary over the land segment.