Wednesday, 3 September 2014

Land use and water quality

MSc student, Alejandra Barrios Rivera (York University) has just completed her time with us, exploring the relationship between land use and water quality in the three headwater catchments of the Water Friendly Farming project.  Thanks to Julie Ewald and Nicholas Aebischer at the GWCT headquarters in Fordingbridge for providing support with GIS and stats.  The data Alejandra used (provided by the Freshwater Habitats Trust) were from the February and March 2013 sampling period and comprised 94 samples from stream sites and 51 from ditches.  This 'should' have been a wet time of year, enabling us to examine the impact of runoff from land, but as it happened, rainfall was 68% lower than usual and so the sampling period is more representative of lower flow conditions.
The three catchments of the Water Friendly Farming project, showing major land uses
Total nitrogen and total phosphorus concentrations in the three catchments were broadly similar.  For both total nitrogen and total phosphorus, upslope woodland area reduced concentrations in water.  For total nitrogen, the upslope arable area was associated with higher concentrations, while for total phosphorus it was the upslope urban area that was associated with higher values. Nitrogen tended to be present at higher concentrations high in the catchments, while for phosphorus, highest concentrations were towards the base of the catchment. Neither slope nor soil permeability influenced nutrient concentrations in water, as might be expected for the relatively low rainfall conditions at the time of sampling.

These findings reflect other results from the project and from our other local monitoring, revealing a high impact of local residents on water quality through discharges from septic tanks and small rural sewage treatment works.  Nitrogen was associated with arable land but as elevated concentrations were recorded high in the catchments where settlements tended to be located, there may also have been a contribution from domestic sources.  We will need to examine data collected under wetter conditions to understand better the agricultural contribution under higher flow conditions when we know that sediment and phosphorus delivery to water from arable land is higher.

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