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Fall 2004 - Vol. 5/No. 1
With loss of ag wavier looming, SAFS researchers shed light on runoff dilemmas
by Sam Prentice, Aaron Ristow and Will Horwath
Much has been done since the federal Clean Water Act took effect in 1972 to mitigate end-of-pipe point source
pollution (PSP). Much attention to assessing and mitigating non point sources of pollution (NPSP), including
groundwater and surface water discharges from industrial, municipal and logging activities, has been done since
the Act’s inception. Noticeably lacking in early efforts is the assessment of NPSP from agricultural land,
which historically has been exempted from the Clean Water Act under a clause known as the ““ag waiver.”
In California, however, the Porter Cologne Water Quality Act does regulate agricultural runoff, and local
institutions have recently implemented monitoring programs aimed at limiting water quality degradation from
agricultural sources.
In January 2004, the shift toward NPSP management led to the elimination of individual ag waivers. By 2005,
agricultural operations must comply with comprehensive new water quality regulations, embodied in a new
“conditional waiver” enacted by the Central Valley Regional Water Quality Control Board (Water
Board). The conditional waiver covers all irrigated farmland in the Central Valley. The Water Board, which is
charged with developing and enforcing new NPSP standards, has presented three options to regional growers:
join a regional water quality coalition group and apply for a group discharge waiver; apply for an individual
waiver; or submit a complete application for a permit. The majority of growers appear to be choosing the first
option, in which coalition group members are held jointly accountable for pollutants coming off of their
collective land.
Developing solutions
The shift toward ambient water quality standards necessitates a rethinking of methodologies on monitoring.
By definition, NPSP impairments are spatially and temporally diffuse, making them difficult to identify,
quantify, and regulate. Analyses of pollutant loading based on protection of beneficial uses as described
in the Basin Plan and Total Maximum Daily Loads (TMDL) require some form of watershed-scale monitoring that
captures transport processes and relates them to land use practices.
The model currently adopted by some groups places downstream TMDL monitoring at its core; in this model,
corrective actions are triggered when downstream pollutant loading exceeds (as yet undetermined) thresholds.
While this model satisfies the legality of NPSP monitoring, technically it requires backtracking excessive
pollutant loads upstream – a laborious process in which it is difficult to quickly identify pollution “hot
spots” within the watershed.
To achieve effective NPSP reductions, however, the development of conceptual
models that correlate water inputs and load reductions with progressive agricultural management practices would
be useful and would provide the credible scientific information required to extend these models into the realm
of ambient water quality standards and monitoring protocols.
Promising SAFS data
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Table 1. Runoff Values from CC and non-CC Farm Fields (2003-04 Rain Season) |
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Non Cover Cropped |
Cover Cropped |
| Total Precipitation Discharged as Runoff |
16.3% |
0.9% |
| Average Suspended Solid Conc. |
2.14g/L |
0.58g/L |
| Average Runoff Velocity |
0.52m/s |
0.24m/s |
One year ago, a SAFS research team implemented the first large-scale, long-term, replicated field trials
aimed at addressing the universal concern of growers statewide: the loss of the agricultural discharge waiver.
Using state-of-the-art flow monitoring equipment, the SAFS team has collected extensive data on both winter and
summer runoff events from agricultural fields in Yolo County. Our efforts include determining flow velocities and
volumes and water quality indicators including organic and inorganic nitrogen and phosphorous, dissolved organic
carbon, turbidity and suspended sediment.
Initial results show a stark contrast in runoff quality and quantity between a field planted in a winter legume/oats
cover crop and a field with no plant cover (see Table 1). Notably, the quantity of runoff discharged from a winter
covercropped field was less than one-tenth the runoff of winter-fallowed fields, or about 1% of total rainfall. In
addition, yields between treatments showed no significant difference. The results support previous research showing
increased infiltration and decreased nutrient losses through the use of cover crops. We are also examining practices
that take advantage of residual plant cover, such as reduced tillage practices, as means to reduce runoff and NPSP.
We believe these practices may be as effective as cover crops in meeting Water Board requirements.
In addition to analyzing nutrient and sediment loading, the SAFS water quality project team is determining
relationships between rainfall intensity and runoff under winter cover-cropped and fallowed fields. Using
minute-by-minute resolution of flow velocities, project hydrologists Wes Wallender and Bellie Sivakumar aim to build
and refine models that predict the response of a particular cropping system, soil type, slope, drainage area, or
other field-level parameters to rainfall intensity. Such models will aid farmers and policy makers in evaluating
management practices and policies that conserve water and soil resources.
Significant impact
Overall, the influence of alternative management practices such as reduced tillage and cover cropping can have a
significant positive impact on runoff quantity and quality. Our initial findings suggest that coalition groups can
meet (and perhaps even exceed) their legal responsibilities to NPSP mitigation by encouraging their grower-members
to adopt some aspect of alternative practices discussed here. Technical pitfalls that accompany winter and summer
season monitoring provide an even greater incentive to focus on preventative solutions, which avoid TMDL violations.
During the second and third years of the CALFED water quality project, we will continue to analyze the effects of
tillage and winter cover crops, focusing on relationships among rainfall intensity, runoff flow velocities, and
nutrient loading potentials. In keeping with the SAFS research paradigm, the economic feasibility of using
alternative practices in California row crop agriculture will be an essential component. Ultimately, this project
aims to develop a “toolbox” for growers and regulators to predict the effects of multiple land use
management systems at the field and watershed scale.
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