Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: I. Annual nitrogen budgets

Abstract

The hydrological and biogeochemical monitoring of catchments has become a common approach for studying the effect of the evolution of agricultural practices on water resources. In numerous studies, the catchment is used as a "mega-lysimeter" to calculate annual input-output budgets. However, the literature reflects two opposite interpretations of the trends of nitrate concentration in streamwater. For some authors, essentially in applied studies, the mean residence time of leached nitrate in shallow groundwater systems is much less than one year and river loads reflect annual land use while for others, nitrate is essentially transport limited, independent of soil nitrate supply in the short term and annual variations reflect changes in climatic conditions. This study tests the effect of agricultural land-use changes on inter-annual nitrate trends on stream water of six small adjacent catchments from 0.10 to 0.57 km² in area, on granite bedrock, at Kerbernez, in Western Brittany (France). Nitrate concentrations and loads in streamwater have been monitored for nine years (1992 to 2000) at the outlet of the catchments. An extensive survey of agricultural practices from 1993 to 1999 allowed assessment of the nitrogen available for leaching through nitrogen budgets. For such small catchments, year-to-year variations of nitrate leaching can be very important, even when considering the 'memory effect' of soil, while nitrate concentrations in streamwater appear relatively steady. No correlation was found between the calculated mean nitrate concentration of drainage water and the mean annual concentration in streams, which can even exhibit opposite trends in inter-annual variations. The climatic conditions do not affect the mean concentration in streamwater significantly. These results suggest that groundwater plays an important role in the control of streamwater nitrate concentration.

What can flux tracking teach us about water age distribution patterns and their temporal dynamics?

“…Rather, the contaminant will also remain in the system and cause continuous low level contamination of the stream for long time periods (e.g. Molenat and GascuelOdoux, 2002;Ruiz et al, 2002a). Therefore, the long tails of p T|Q and p F suggest that the near-chemostatical behavior observed in a considerable range of catchments (e.g.…”

Hydrological hysteresis and its value for assessing process consistency in catchment conceptual models

“…Mean annual drainage is 360 mm (±60 %) at the outlet. There is a high water deficit in the annual budget almost every year due to underflows below the outlet (Ruiz et al, 2002). The catchment lies under granite (leucogranodiorite of Plomelin), the upper part of which is weathered from 1 to more than 20 m deep.…”

Spatial distribution of soils determines export of nitrogen and dissolved organic carbon from an intensively managed agricultural landscape

“…Despite this, higher NO − 3 -N concentrations were exhibited in the northern branch during summer 2009. As pointed out in many studies (Böhlke and Denver, 1995;Ruiz et al, 2002a;Worrall and Burt, 2001), soils are able to store N, leading to poor correlations between N losses from agricultural soils and headwater quality. Consequently, N leaching is affected by past land use and management practices (Hansen et al, 2011(Hansen et al, , 2012, which varied a lot in this area during the past 300 yr (Caspersen and Fritzbøger, 2002), but a further study of this aspect was out the scope of the present study.…”