Page 191 - Biosystems Engineering

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quality standards. TMDLs include both point source and NPS dis-
charges that arise from a watershed or the environs of a watercourse
(Ward and Benaman 1999). The CWA requires development of TMDLs
for all waters on the 303(d) list by developing restoration scenarios.
The creation of this list is authorized by Section 303(d) of the CWA.
The ultimate goal of a TMDL development and implementation can
be stated as removal of bodies of water from the 303(d) list by attain-
ing water quality standards. Eventually, the list of impaired bodies of
water and established TMDLs compiled by states, territories, and
authorized tribes must be approved by USEPA.
These requirements made by USEPA lead to the assessment of
NPS water quality problems and identification of critical contributing
areas in several states. For example, in the U.S. state of Pennsylvania,
Hamlett et al. (1995) identified and ranked critical NPS contributing
areas on a watershed-by-watershed basis for the entire state. In Vir-
ginia, Tim et al. (1992) used an integrated approach, coupling a water
quality computer simulation model and the Virginia Geographic
Information System (GIS) to delineate critical areas of NPS pollution
in the Nomini Creek watershed located in Westmoreland County, Vir-
ginia. Evans and Myers (1990) developed a methodology using GIS
and DRASTIC to evaluate groundwater pollution potential in regional
areas larger than 13,000 ha. DRASTIC is a widely used ground-water
vulnerability mapping method. DRASTIC is named for the seven fac-
tors considered in the method: Depth to water, net Recharge, Aquifer
media, Soil media, Topography, Impact of vadose zone media, and
hydraulic Conductivity of the aquifer (Aller et al. 1987).

5.2 Watershed Models

5.2.1 Need for Watershed Models
The CWA also required states to develop management plans to reme-
diate NPS pollution problems. With limited resources available (in
terms of time, labor, and money), it is imperative that control and
implementation programs focus on critical contributing areas and
adequately consider the impacts of alternative management, land
use, and conservation approaches (e.g., conservation tillage, contour
cropping, strip cropping, and fertilizer management) on NPS pollution.
Evaluating NPS pollutant reduction effectiveness of alternative man-
agement, land use, and conservation practices at a watershed scale
through experiments and monitoring systems is not feasible because
of the enormous cost, time, and labor involved. Modeling studies
based on experimental data are often the only viable means of providing
timely inputs to management decisions with the least cost. Therefore,
modeling NPS pollutant fate and transport processes at a watershed
scale is fundamental to addressing contamination of surface and
ground waters. Watershed-scale NPS models are currently used for a