An investigation of runoff and quality responses on an agricultural catchment with specific reference to storm events : the River Wallington, Hampshire

The primary objective of this study is to investigate the runoff and quality variations exhibited on a medium-sized, agricultural catchment, dominated by a chalk and clay geology: the River Wallington, Hampshire, southern England. Emphasis has been focused on storm-based responses, particularly quality variations, monitored at two locations between October 1981 and May 1983. Quality parameters monitored include nitrate-nitrogen, phosphate, potassium, pH, chloride, sodium, suspended, volatile and dissolved solids, conductivity, temperature and dissolved oxygen. Stable rainfall and runoff conditions were observed with mean daily flows ≤ 0.2 m^3/s. Bankfall discharges of 10.0 m^3/s are exceeded at least once a year. Soil moisture status and antecedent catchment conditions are important factors in modelling observed runoff. Storm-based rainfall-runoff responses are adequately modelled using unit hydrograph techniques on seasonally divided data, with observed peak discharges estimated to within 10% and time to peak flows to within 2 hours. Definition of effective precipitation is a prime control on the performance of this prediction, particularly during the summer when more variable antecedent conditions occur. Loss of surface water as recharge to the Chalk aquifer complicates summer modelling. Rock and soil type are the major controls on surface water quality variations, which show stable responses during the study period. The influence of the Chalk maintains high pH, T.D.S., S.E.C. and NO_3-N concentrations, with long-term trends in the Chalk groundwater showing rising NO_3-N and Cl concentrations. Baseflow concentrations show minor dilution with higher discharges while stormflow concentrations show a wide range of behavioural responses with increasing flows, including dilution, concentration, flushing, between sample variability, and stable concentrations. The goodness-of-fit shown by bivariate concentration-discharge models used to describe the data are generally poor, indicating the importance of controls other than discharge in determining runoff quality. Mean storm concentrations show little change from the mean baseflow concentrations with orders of magnitude during storms typically: T.D.S. > V.S. > S.S. > Cl > Na > NO_3-N > D.O. > K > PO_4. The negligible concentration changes result in an increase in loads transferred during storms. The magnitude of increase depends on storm discharges but is typically double for solutes and fifty-fold for particulates. Storms are therefore important mechanisms of material transfer. (D72033/87)