Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: II. Temporal variations and mixing processes

Abstract. In catchments with impervious bedrock, the nitrate concentrations in streamwater often show marked seasonal and small inter-annual variations. The inter-annual trends are usually attributed to changes in nitrogen inputs, due to changes in land use or in nitrogen deposition whereas seasonal patterns are explained in terms of availability of soil nitrate for leaching and of seasonality of nitrogen biotransformations. The companion paper showed that inter-annual variations of nitrogen in streamwater are not directly related to the variations of land use. The aim of this study is to describe nitrate concentration variations in a set of very small adjacent catchments, and to discuss the origin of the inter-annual and seasonal trends. Data from four catchments at the Kerbernez site (South Western Brittany, France) were used in this study. Nitrate concentrations in streamwater were monitored for eight years (1992 to 1999) at the outlet of the catchments. They exhibit contrasting inter-annual and seasonal patterns. An extensive survey of agricultural practices during this period allowed assessment of the amount of nitrogen available for leaching. The discharges measured since 1997 show similar specific fluxes but very different seasonal dynamics between the catchments. A simple, lumped linear store model is proposed as an initial explanation of the differences in discharge and nitrate concentration patterns between the catchments. The base flow at the outlet of each catchment is considered as a mixture of water from two linear reservoirs with different time constants. Each reservoir comprises two water stores, one mobile contributing to discharge, the other, immobile, where nitrate moves only by diffusion. The storm flow, which accounts for less than 10% of the annual flux, is not considered here. Six parameters were adjusted for each catchment to fit the observed data: the proportion of deep losses of water, the proportion of the two reservoirs and the size and initial concentration of the two immobile stores. The model simulates the discharge and nitrate concentration dynamics well. It suggests that the groundwater store plays a very important role in the control of nitrate concentration in streamwater, and that the pattern of the seasonal variation of nitrate concentration may result from the long term evolution of nitrogen losses by leaching. Keywords: nitrate, diffuse pollution, groundwater, seasonal variations, agricultural catchment, simulation model

[1]  Lawrence E. Band,et al.  Exploring functional similarity in the export of Nitrate‐N from forested catchments: A mechanistic modeling approach , 1998 .

[2]  A. Edwards,et al.  Factors influencing dissolved nitrogen concentrations and loadings in upland streams of the UK , 1995 .

[3]  Véronique Beaujouan,et al.  A nitrogen model for European catchments: INCA, new model structure and equations , 2002 .

[4]  Véronique Beaujouan,et al.  Modelling the effect of the spatial distribution of agricultural practices on nitrogen fluxes in rural catchments , 2001 .

[5]  Spectral and cross-spectral analysis of three hydrological systems , 2000 .

[6]  E. J. Wilson,et al.  A semi-distributed integrated flow and nitrogen model for multiple source assessment in catchments (INCA): Part II — application to large river basins in south Wales and eastern England , 1998 .

[7]  Alan R. Hill,et al.  Nitrate Removal in Stream Riparian Zones , 1996 .

[8]  James W. Kirchner,et al.  Sodium and chloride levels in rainfall, mist, streamwater and groundwater at the Plynlimon catchments, mid-Wales: inferences on hydrological and chemical controls , 2000 .

[9]  Véronique Beaujouan,et al.  Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: I. Annual nitrogen budgets , 2002 .

[10]  Lawrence E. Band,et al.  Regulation of Nitrate‐N Release from Temperate Forests: A Test of the N Flushing Hypothesis , 1996 .

[11]  P. Johnes,et al.  Nitrate in surface waters. , 1993 .

[12]  H. Pauwels.  Natural Denitrification in Groundwater in the Presence of Pyrite: Preliminary Results Obtained at Naizin (Brittany, France) , 1994 .

[13]  Tim Burt,et al.  Temporal and spatial patterns of nitrate losses from an agricultural catchment , 1987 .

[14]  I. Burns,et al.  A MODEL FOR PREDICTING THE REDISTRIBUTION OF SALTS APPLIED TO FALLOW SOILS AFTER EXCESS RAINFALL OR EVAPORATION , 1974 .

[15]  Richard P. Hooper,et al.  Modelling streamwater chemistry as a mixture of soilwater end-members — A step towards second-generation acidification models , 1990 .

[16]  V. Chaplot,et al.  Nitrate Depletion During Within-Stream Transport: Effects of Exchange Processes Between Streamwater, the Hyporheic and Riparian Zones , 2000 .

[17]  M. Neill.  Nitrate concentrations in river waters in the south-east of Ireland and their relationship with agricultural practice , 1989 .

[18]  Patrick Durand,et al.  Mechanisms of Nitrate Transfer from Soil to Stream in an Agricultural Watershed of French Brittany , 2002 .

[19]  K. Bencala,et al.  Overview of a simple model describing variation of dissolved organic carbon in an upland catchment , 1996 .

[20]  T. Burt,et al.  Inter-annual controls on nitrate export from an agricultural catchment — how much land-use change is safe? , 2001 .

[21]  D. Walling,et al.  Stream nitrate levels in a small catchment in South West England over a period of 15 years (1970-1985) , 1988 .

[22]  E. J. Wilson,et al.  A semi-distributed ntegrated itrogen model for multiple source assessment in tchments (INCA): Part I — model structure and process equations , 1998 .

[23]  A. Mariotti.  La de´nitrification dans les eaux souterraines, principes et me´thodes de son identification: Une revue , 1986 .

[24]  J. Kirchner,et al.  Fractal stream chemistry and its implications for contaminant transport in catchments , 2000, Nature.

[25]  P. J. Wigington,et al.  Use of factor analysis to investigate processes controlling the chemical composition of four streams in the Adirondack Mountains, New York , 1996 .

[26]  G. Roberts,et al.  Nitrate leaching from a small catchment in central England , 1996 .

[27]  W. Dodds,et al.  Spatial and temporal patterns of nitrogen concentrations in pristine and agriculturally-influenced prairie streams , 2001 .