Nitrate Reduction in a Groundwater Dominated Catchment: How Good Are Our Models?
Geological Survey of Denmark and Greenland (GEUS)
The nitrate load from agricultural land to lakes and coastal water in Denmark has, during the past 20 years, been reduced by about 50 percent by government regulations imposed on agricultural practice. The EU Water Framework Directive (WFD) requires that good ecological status be achieved for all water bodies by 2015 and prescribes that surface water and groundwater resources are managed in an integrated context. This will require an additional reduction of nitrate load by 50 percent, which economically will be very painful for the agricultural sector. The regulations imposed until now have been general, i.e. the same restrictions for all areas independent on the subsurface conditions. Studies have shown that on a national basis about 2/3 of the nitrate leaching from the root zone is reduced naturally in the subsurface before reaching the streams. This implies that if a general agricultural regulation reduces nitrate leaching by 100 kg N, the nitrate load to surface water will only be reduced by 33 kg N. Therefore it is much more cost-effective to identify robust areas, where nitrate leaching through the root zone is reduced in the saturated zone before reaching the streams, and vulnerable areas, where no subsurface reduction takes place, and then only impose restrictions on the vulnerable areas. Distributed hydrological models can make predictions at grid scale, i.e. at much smaller scale than the full catchment. Hence these models have a potential for being able to differentiate between robust and vulnerable areas. However, in all previous studies we have seen, distributed models do not have predictive capability at scales much smaller than the catchment scale. A constraint in this respect is that distributed models often do not include local scale hydrogeological heterogeneities that are known to be important for reactive transport. NICA (Nitrate reduction in geologically heterogeneous catchments) is a new research project that will develop tools for assessing nitrate reduction in the subsurface between the root zone and the streams and methodologies for assessing at which spatial scales such tools have predictive capabilities (see the enclosed figure). A new instrument will be developed for airborne geophysical measurements, MiniSkyTEM, dedicated for identifying geological structures and heterogeneities in the upper 30 m. State-of-the-art hydrological models (DAISY, MIKE SHE/MIKE11, HydroGeoSphere, RWHET) will be applied and the effect of geological heterogeneity will be analyzed by use of stochastic geological realizations such as TProGS. A new concept, Representative Elementary Scale (RES), will be developed for assessing the minimum scale at which models, with a given data input, potentially have predictive capabilities. The studies will be conducted in a 10 km2, densely instrumented catchment and tested in a 101 km2 catchment, where farmers and authorities will be actively involved in evaluating possible measures for reducing the nitrate load to surface water in a cost-effective manner. The economic gain from a cost-efficient location of the measures will be evaluated. The talk will present the policy background, the methodology, the case areas and preliminary results.