Modeling effectiveness of two runoff mitigation measures in the Netherlands.

Rainfall that exceeds the soil's maximum infiltration rate is prone to runoff, and the excess rainfall will flow toward open water systems. Nutrients, pesticides or other contaminants may be transported along with this overland flow, thus contaminating surface waters. There are various measures that can be implemented to prevent or reduce runoff, which involve either improving the soil's infiltration capacity or temporarily storing more water at the field scale. The aim of this study was to determine the effectiveness of two mitigation measures, i.e., micro-dams and edge-of-field trenches, in reducing the total number of runoff events and the runoff volume for specific rainfall events. For this purpose, numerical simulations were performed with a deterministic soil-water-atmosphere-plant model for reference situations and for situations involving either of the two mitigation measures. The mitigation measures are implemented as a change in the ponding threshold height above which the model predicts runoff. For this purpose, we considered several soil / groundwater level / crop / intrinsic field soil surface storage situations that are common in the Netherlands. For ridge-furrow cropping systems, micro-dams are more effective than edge-of-field trenches. Depending on the soil type (excluding sand), the minimum effectiveness is 70% and may be >90% in specific situations. For the edge-of-field trench, the reduction in runoff events was mostly in the 24-35% range, while the effectiveness for the runoff volume for a rainfall event that typically occurs once per year was in the 13-48% range (excluding sand). Due to the relatively high hydraulic conductivity at saturation for the sandy soils, runoff was simulated in only a few cases for these soils. The effectiveness was evidently dependent on intrinsic field soil surface storage and soil types, varied slightly between crop types and was very similar across the groundwater level classes considered.

Performance assessment of nitrate leaching models for highly vulnerable soils used in low-input farming based on lysimeter data.

The agricultural sector faces the challenge of ensuring food security without an excessive burden on the environment. Simulation models provide excellent instruments for researchers to gain more insight into relevant processes and best agricultural practices and provide tools for planners for decision making support. The extent to which models are capable of reliable extrapolation and prediction is important for exploring new farming systems or assessing the impacts of future land and climate changes. A performance assessment was conducted by testing six detailed state-of-the-art models for simulation of nitrate leaching (ARMOSA, COUPMODEL, DAISY, EPIC, SIMWASER/STOTRASIM, SWAP/ANIMO) for lysimeter data of the Wagna experimental field station in Eastern Austria, where the soil is highly vulnerable to nitrate leaching. Three consecutive phases were distinguished to gain insight in the predictive power of the models: 1) a blind test for 2005-2008 in which only soil hydraulic characteristics, meteorological data and information about the agricultural management were accessible; 2) a calibration for the same period in which essential information on field observations was additionally available to the modellers; and 3) a validation for 2009-2011 with the corresponding type of data available as for the blind test. A set of statistical metrics (mean absolute error, root mean squared error, index of agreement, model efficiency, root relative squared error, Pearson's linear correlation coefficient) was applied for testing the results and comparing the models. None of the models performed good for all of the statistical metrics. Models designed for nitrate leaching in high-input farming systems had difficulties in accurately predicting leaching in low-input farming systems that are strongly influenced by the retention of nitrogen in catch crops and nitrogen fixation by legumes. An accurate calibration does not guarantee a good predictive power of the model. Nevertheless all models were able to identify years and crops with high- and low-leaching rates.

Effectiveness of unfertilized buffer strips for reducing nitrogen loads from agricultural lowland to surface waters.

Unfertilized buffer strips (BS) are widely accepted to reduce nitrogen (N) loads from agricultural land to surface water. However, the relative reduction of N load or concentration (BS effectiveness, BSE), varies with management and local conditions, especially hydrogeology. We present novel experimental evidence on BSE for 5-m-wide grass BS on intensively drained and managed plain agricultural lowland with varying hydrogeology. We selected characteristic sites for five major hydrogeological classes of the Netherlands and installed paired 5-m-wide unfertilized grass (BS) and reference (REF) treatments along the ditch. The REF was managed like the adjacent field, and BS was only harvested. Treatments were equipped with reservoirs in the ditch to collect and measure discharge and flow proportional N concentration for 3 or 4 yr. In addition, N concentration in upper groundwater was measured. We found a statistically significant BSE of 10% on the peat site. At the other sites, BSE for N was low and statistically insignificant. Low BSE was explained by denitrification between adjacent field and ditch, as well as by the site-specific hydrologic factors including low proportion of shallow groundwater flow, downward seepage, low residence time in the BS, and surface runoff away from the ditch. We emphasize that a REF treatment is needed to evaluate BSE in agriculture and recommend reservoirs if drainage patterns are unknown. Introduction of a 5-m-wide BS is ineffective for mitigating N loads from lowland agriculture to surface waters. We expect more from BS specifically designed to abate surface runoff.

A novel method to determine buffer strip effectiveness on deep soils.

Unfertilized buffer strips (BS) generally improve surface water quality. High buffer strip effectiveness (BSE) has been reported for sloping shallow aquifers, but experimental data for plain landscapes with deeply permeable soils is lacking. We tested a novel method to determine BSE on a 20-m-deep, permeable sandy soil. Discharge from soil to ditch was temporarily collected in an in-stream reservoir to measure its quantity and quality, both for a BS and a reference (REF) treatment. Treatments were replicated once for the first, and three times for the next three leaching seasons. No significant BSE was obtained for nitrogen and phosphorus species in the reservoirs. Additionally, water samples were taken from the upper groundwater below the treatments. The effect of BS for nitrate was much bigger in upper groundwater than in the reservoirs that also collected groundwater from greater depths that were not influenced by the treatments. We conclude that measuring changes in upper groundwater to assess BSE is only valid under specific hydrogeological conditions. We propose an alternative experimental set-up for future research, including extra measurements before installing the BS and REF treatments to deal with spatial and temporal variability. The use of such data as covariates will increase the power of statistical tests by decreasing between-reservoir variability.