Journal of Conference Abstracts

Links Between Runoff Generation, Temperature and Nitrate-N Leaching from Forested Catchments

Lotta Andersson¹ (lotan@tema.liu.se) & Ahti Lepistö² (ahti.lepisto@vyh.fi)

¹ Department of Water and Environmental Studies, Linköping University, S-581 83 Linköping, Sweden.

² Finnish Environment Institute, P.O. Box 140, FIN-0025 Helsinki, Finland.

The objective of this study is to improve the understanding of hydroclimatological factors affecting NO₃^- flow from shallow boreal forested soils. The ability to predict dynamics of NO₃^- flow is important, as it leads to the potential to predict impacts of imposed environmental changes, i.e., increased N-deposition (e.g., Lepistö, 1995), climatic change (e.g., Wright & Schindler, 1995) and forest management (e.g., Swank & Johnson, 1994).

Nitrate-N leaching is affected by flow paths and transit times. Saturated contributing areas may have a potential to regulate nutrient fluxes between upland areas and the stream. It is therefore hypothesised that the extension of contributing areas affects the amount of NO₃^- leached to the stream (Lepistö, 1994). In the beginning of a flow increase, intensive leaching of NO₃^-, accumulated in surficial soil layers, may occur (Stoddard, 1994, Andersson et al., 1994, Creed et al., 1996). Seasonal variability of temperature regulating biological uptake is another climate-related factor that has to be considered (e.g., Arheimer et al., 1996).

One catchment in Sweden (1994-1996) and one in Finland (1991-1995) were intensively monitored in terms of NO₃^--N concentrations in surface and groundwater, and hydrological dynamics. The catchments represent hydrologically quickly responding shallow till soils with open bedrock areas. Short-term dynamics of NO₃^--N concentrations and ¹⁸O contents were monitored during snowmelt and intensive rain periods, as well as within-year variation. Shallow and deep groundwater was also sampled from several points in the Swedish catchment and from one transect in the Finnish catchment. Areal distribution and temporal dynamics of soil moisture and groundwater levels were monitored, and streamflow was registered continuously from the outlets and also from a spring in the Swedish catchment.

Isotope techniques, using ¹⁸O, were applied to estimate the extent of event water contributing areas and the direct runoff fraction. The extensions of saturated contributing areas, estimated by the semidistributed Topmodel, were compared with those estimated by isotopic methods. Differences in the assumptions made by the model and in the isotopic hydrograph separation must, however be taken into consideration (Holko and Lepistö, 1997). Dynamics of groundwater and soil water contents were simulated by Topmodel.

On a short term (hour-day) basis, links between streamflow generation and NO₃^- flushing were tested. As indicators, the dynamics of soil water contents and groundwater levels, and the fraction of direct runoff were used. Soil water contents were obtained from measurements and model simulations, the fraction of direct runoff from isotope studies and modeling. In addition, the effect of a groundwater increase into previously unsaturated parts of a N-saturated N-profile was studied, using monitored and modeled groundwater levels. Biological activity, as a function of air temperature, was taken into account explaining the seasonal variability.

Furthermore, a two-component model (Lepistö, 1994) was tested to calculate monthly exports, assuming that all the NO₃^--N deposition on saturated, contributing areas is flushed relatively quickly to the streams, while nitrate N deposited on non-contributing areas is infiltrated to the soil layers or retained by the biomass.

References

Andersson, L., Arheimer, B., Sundblad, K. & Lepistö, A. in Flow regimes from experimental and network data. (eds. Seuna, P. et al.) 399-407 (IAHS Publ. 221, IAHS press Wallingford, UK, 1994).

Arheimer, B., Andersson, L., Lepistö, A. Journal of Hydrology 179, 281-304. (1996).

Creed, I.F., Band, L.E., Foster, N.W, Morrison, I.K., Nicolson, J.A., Semkin, R.S. & Jeffries, D.S. Water Resources Research 32, 3337-3354 (1996).

Holko, L. & Lepistö, A. Journal of hydrology (in press)

Lepistö, A. Journal of Hydrology 171, 103-123 (1995).

Lepistö, A. Aqua Fennica 24, 103-120 (1994).

Stoddard, J.L. in Environmental chemistry of lakes and reservoirs (ed. Baker, L.A.) 237-231. (American Chemical Society, Washington DC. Advances in Chemistry Series No. 237, 1994).

Swank, W.T. & Johnson, C.E. in Biogeochemistry of small catchments: A tool for environmental research (eds. Moldan, B. & Cerny´, J.) (SCOPE 51. 419 p, 1994).

Wright, R.F., Schindler, D.W. Water Air and Soil Pollution 85, 89-99 (1995).