Environmental Engineering Reference
In-Depth Information
3
Modeling methods
is a parameter in groundwater-flow models that
solve for hydraulic head (i.e. groundwater level).
Recharge estimates can be obtained through a
model calibration process in which recharge
and other model parameter values are adjusted
so that simulated water levels agree with meas-
ured water levels. The simulation that provides
the closest agreement is called the best fit, and
the recharge value used in that simulation is
the model-generated estimate of recharge.
Both diffuse and focused recharge can be
estimated with models. When selecting a model
it is important to ensure that assumptions
inherent in the model are consistent with the
user's conceptual model of the recharge pro-
cess. As with other estimation methods, space
and time scales of recharge estimates vary
for different models and model applications.
Applications of watershed or groundwater-flow
models may assume that recharge is uniform
over the simulated domain, or recharge may be
allowed to vary in space within the domain. In
terms of time scales, recharge can be estimated
on a daily, monthly, or annual basis; or recharge
may be assumed constant over time.
This chapter provides descriptions of generic
categories of models: unsaturated zone water-
budget models, watershed models, and ground-
water-flow models. Models within each category
share a common approach, but complexity and
features of models can vary substantially. An
overview of models within each category is pre-
sented, representative models are described,
and major assumptions underlying the models
3.1 Introduction
Simulation models are widely used in all types
of hydrologic studies, and many of these models
can be used to estimate recharge. Models can
provide important insight into the functioning
of hydrologic systems by identifying factors that
influence recharge. The predictive capability of
models can be used to evaluate how changes in
climate, water use, land use, and other factors
may affect recharge rates. Most hydrological
simulation models, including watershed mod-
els and groundwater-flow models, are based
on some form of water-budget equation, so
the material in this chapter is closely linked
to that in
Chapter 2
. Empirical models that
are not based on a water-budget equation have
also been used for estimating recharge; these
models generally take the form of simple esti-
mation equations that define annual recharge
as a function of precipitation and possibly other
climatic data or watershed characteristics.
Model complexity varies greatly. Some
models are simple accounting models; others
attempt to accurately represent the physics of
water movement through each compartment
of the hydrologic system. Some models provide
estimates of recharge explicitly; for example,
a model based on the Richards equation can
simulate water movement from the soil sur-
face through the unsaturated zone to the water
table. Recharge estimates can be obtained indir-
ectly from other models. For example, recharge
