Contrasted Long-Term Evolutions of Nitrate Concentrations in Soils, Groundwater and Streams in a Network of Small Experimental Watersheds in Western Europe

Despite the large amount of research in the past 30 years dedicated to understand and model the dynamics of agricultural-borne diffuse pollution (mainly nitrate) at the watershed level, there is no accepted perceptual model explaining simultaneously the observed contrasted dynamics of solutes in the soil, the recharge, the groundwater and the streams. All of these compartments are responsible for buffering the signal entering from agricultural inputs, which is itself difficult to measure accurately. Recently, the analysis of long-term data sets concerning water quality in streams has allowed improving significantly our understanding of the average response time of watersheds to historical agricultural inputs. However, as historical agricultural practices are usually poorly documented, large assumptions are needed to achieve such exercises. Lastly, long-term monitoring of groundwater quality is usually lacking or poorly representative spatially. In this communication, we will present results from a long-term comprehensive monitoring of agricultural inputs and surface water (20 years) and groundwater chemistry (10 years) in a set of small headwater watersheds (ORE AgrHys, http://www.inra.fr/ore_agrhys/). We will focus on two neighbouring watersheds where data from a dense piezometric network is available. Results showed (i) a strong stability in the stream chemistry whereas agricultural inputs in these small watersheds were highly variable from year to year, (ii) a high spatial heterogeneity of the groundwater chemistry, both laterally along the hillslope and vertically and (iii) contrasted behaviour of long-term trends in agricultural inputs and nitrate concentration in groundwater. A relatively simple model, based on linear reservoirs, was calibrated on the available data set using weather data and agricultural practices as forcing variables. The model accounted well for the observed seasonal variations and long-term trends observed in nitrate concentrations in streams, and for the long term trends in groundwater. Inversion of the model using weather data from 1950 to 2011 allowed estimating the changes in agricultural inputs since the beginning of agriculture intensification in the region. The reconstructed time series compared well with existing long-term stream water quality monitoring in larger watersheds in the same region. Discussion will focus on the implications of these results in term of solute pathways and nitrate residence times along the hillslope and on the potential of this approach for predicting the residence time of nitrate in watersheds where only few years of monitoring are available.