It is a physically-based, comprehensive computer model that was well-suited to provide a landscape-level assessment of nitrate leaching. It is based on over 30 years of research and is internationally accepted as a robust interdisciplinary modeling tool set with over 3,500 published journal articles.  Its spatial and temporal resolutions are useful in meeting MPEP objectives: 1) it has a daily time step which allows for the capture of daily management decisions, and 2) its sub-field spatial resolution provides a desirable level of detail. The modeling process is driven by the water balance, considers crop growth, nutrient and water uptake, local climate, local soil properties​, and management practices, which provides a mechanism for comparing management scenarios.

The SWAT model required adaptation to properly represent conditions in California agriculture and meet ILRP program requirements. The work was done in collaboration with NRCS and UC and funded in part by $2M CIG grant. CV-SWAT is the result of this work and is the assessment tool used in the MPEP. CV-SWAT includes over 50 calibrated crop models covering over 98% of Central Valley acreage and the main components of the model are described below:

Yes. The most detailed soil data that is readily available comes from NRCS’ soil surveys, which are available digitally. Climatic information is similarly available from the California Irrigation Management Information System (CIMIS) and other sources. This allows site-specific data on growing conditions to be efficiently incorporated into landscape-level (considering many, diverse fields at once) assessments in computer models. SWAT, A widely used watershed model developed by the USDA Agricultural Research Service, in conjunction with many others, has been adapted for use in California with NRCS as a major supporter. By using site-specific model inputs (soil and climatic information), model outputs (estimates of the amount and quality of water moving through soils and interacting with crops) reflect site-specific conditions to the greatest extent practicable with current technology. 

Landscape-level results will be compared to runs of more detailed, site-specific models and to site-specific field measurements. The landscape-level model will then be adjusted as necessary to improve the accuracy of its estimates. 

The schedule does not allow for complete dependence on new studies, since they take several years to develop and complete. Fortunately, FREP, UCCE, commodities groups, USDA, and many others have been studying efficient use of N fertilizer for years. The focus is on ensuring that this existing knowledge is available and useful to growers, on encouraging or doing high-priority new work, and on using available information to assess how practices affect groundwater. The assessment involves modeling that is rigorously checked against field observations. Where groundwater quality data are needed, observations from existing wells will be used to the maximum extent possible, but some new monitoring wells may also be needed.

The focus of the program is on understanding what happens in the soil and how it relates to management practices. This usually requires using operational and site information, sometimes with measurements in the crop root zone. Monitoring wells are rarely used for this purpose because groundwater quality data does not tell us enough about where a contaminant came from or, if it happened to come from an agricultural field, why it was leached from the soil. A limited number of monitoring wells may need to be installed at research sites specifically developed to monitor groundwater conditions in response to advanced practices.

Some groundwater information will be needed to understand long-term trends in regional groundwater quality. These data may be from samples taken in existing wells managed by others, from those shared with other groups who need to monitor groundwater, and from a few wells that coalitions install to answer a specific question that cannot be addressed another way. The coalitions manage this regional “Groundwater Trend Monitoring” separately from the MPEP. 

Some. The agricultural research community has been working on soil fertility and efficient use of nitrogen for decades, producing a great deal of useful information that is being assimilated into the performance assessment and outreach parts of the program. After all, using existing knowledge gets quicker results, and allows us to focus new studies on pressing questions for which answers are as yet unclear. This approach is consistent with a widely accepted standard for scientific inquiry: begin by understanding what is already known, then frame a new and relevant question before launching into costly experimentation. 

No, other than specific field studies and samples growers take for their own information. It is quicker and more cost effective to work with growers and use data that they already rather than collect new.  

This is one of the most challenging parts of the program and it requires bringing several approaches together to establish this relationship.

First, we must understand what is happening to water in the root zone. Root zones that produce very little leaching nitrate cannot reasonably be assumed to be big sources of nitrate going to groundwater, therefore it is crucial to understand what is happening to water in the root zone, before it moves downward toward groundwater. Aquifers are large and reflect the combined influence of many individual fields. We use a soil-water-crop model to evaluate nitrate leaching and groundwater recharge through cropland as a function of management, crop, and soil characteristics. The results characterize a reasonably accurate representation of the true pattern of water and nitrate emanating from root zones, which is the type of input needed to understand how underlying groundwater will be affected. 

Second, we collaborate with university partners to determine groundwater quality impacts at the landscape level (e.g., township, groundwater sub-basin, or regional). Long-term groundwater quality calculations can be made with new simulation tools that reflect the amount of water and nitrate that are recharging from irrigated lands but also account nitrate and recharge from other sources, and for the legacy effects past nitrate leaching across these landscapes. The results can help to predict future groundwater quality trends and to set appropriate expectations for when improvements in water quality at domestic and public water supply wells might be seen.

Third, groundwater quality trends can be measured directly in existing or new wells, which is another requirement in the Central Valley Irrigated Lands permits. To achieve this goal efficiently, Coalitions are working together and with neighbors and partners to monitor a regional well network.

Yes, the landscape-level model used to assess management practices incorporates other nitrate sources, such as industrial, dairy, and septic systems, so that water quality relationships of irrigated lands are assessed in a realistic context. 

In a sense, yes. In 2019, a Groundwater Protection process was added to Central Valley Irrigated Lands permits, requiring that field-specific N management (applied N and yield) information be considered along with other factors to estimate N leaching from lands in each township. Further, the effects of this leaching on underlying groundwater quality must be assessed, and reductions necessary to achieve water quality objectives need to be determined. Although field-specific information is used in this calculation, the results are to be reported at the township level.

Otherwise, the MPEP is not focused on assessing individual fields, but rather on assessing the performance of classes of management practices, crop, and soil combinations over time. The results of these assessments will be made available to growers to help them to assess their own performance. You will be able to compare the results for several alternative suites of practices, in terms of their effect on N leaching for the specific combination of crops and soils that comprises your field. 

If the exact regime is known for a site, it can be reflected in modeling for that site. So, modeling can be quite specific, and results reflect specific management regimes.  

The modeling to date considers a relatively common multiple application scenario for most drip and microspray irrigated crops. It applies this regime to all fields. The simple reason for this is that nitrogen management summary reports contain no information about nitrogen application other than the amount applied, so there is currently no basis on which to customize application timing for each field. Since something must be specified, a regime that is fairly common and efficient is one of several that is modeled. These management regimes were developed with people knowledgeable about each crop. Methods can be changed in the future as more is learned or as farming methods evolve.

Both have been identified as usable metrics in Irrigated Lands permits, so either can be used. A-R is the more commonly used, representing the pounds of N per acre that is subject to leaching in a given year or season. However, since other things happen to N not taken up by plants, A-R does not reflect leaching as such. Nevertheless, it is widely and correctly accepted as an index of general leaching risk.

The A/R metric, on the other hand, has no units and is not a physical quantity, but rather one way to express efficiency. Growers using 70% of 30 and 300 lb/acre N applications will have the same A/R. A/R is mathematically more erratic and more likely to be high when there is no real problem, or low when there is.

In terms of metrics, the SSJV MPEP tends to emphasize A-R as one way to indicate the level of leaching risk. Use of A/R efficiency indicators is limited to comparing among alternative systems for the same crop. 

Similarities are that applied N and that removed by crops are considered in both, but otherwise the two assessments are very different. The goal of the SBX2-1 assessment is to assess historical N balances, whereas the MPEP must assess the influence of current or possible future management on underlying water quality.

The SBX2-1 was a simple N balance, in which applied N could either be taken up (each crop has an estimated demand), lost as gas (as a fixed proportion of applied N), or leached. This simple balance was calculated for each field each year. This accounting tool is not in the same “family” of physically based, complex watershed models as the SWAT model employed in the MPEP. SWAT is a better fit for the MPEP but was not needed to meet the objectives of SBX2-1. The MPEP assessment links SWAT to NPSAT, a groundwater assessment tool set developed by University of California. SBX2-1 does not contain an explicit groundwater component because it was not needed to achieve its objective. Neither assessment addresses processes between the bottom of the root zone and the water table, other than to assume that recharging water does not change much as it flows through these layers.

The SBX2-1 N rates were developed by estimating the available N sources from regional manure production and fertilizer sales, and then apportioning that total amount onto land in proportion to (but sometimes much greater than) crop needs. The N supply drove that assessment. The MPEP is more demand-driven and assesses N rates in the range from those recommended as agronomically sound (low end) upward through the range reported by growers (sometimes exceeding the strictly agronomic rate). In general, the SBX2-1 rates are in the range of 2.5 to 3 times these rates. 

They are used as a reference point to ensure that the full range of N rates and yields that growers report are considered for each crop. In the Groundwater Protection assessment, they are used to select the N rate/yield combinations to apply to each field within a township. 

Eventually, sufficient detail to capture the combined effects of practices on leaching outcomes will be needed. Coalitions endeavored to select and develop a platform that would enable growers to do just this. Modeling will never be a perfect solution, but it allows us to recognize nuances of management better than any approach except site-specific monitoring, which is prohibitive due to the astronomical cost and difficulty of obtaining reliable monitoring results for vast acreages. 

The Irrigated Lands permits require an assessment, but the method of assessment is up to growers and it was up to their coalitions to develop it. As such, these tools belong to the grower coalitions. How they are used is up to them. Of course, whatever assessment method is used, it is only useful if the results are credible to others. Thus, coalitions have worked to ensure that scientific, regulatory, and stakeholder partners are informed about and have the chance to become comfortable with assessment methods as they have been developed.