Surface Water Quality as Affected by Land Use On the Upstream Area of Drinking-Water Reservoir.

Soil variability in upstream area accounts for the problem of water quality of drinking water reservoir.  The objectives of this study were to examine the impact of land use on the quality of surface water through incorporating a year round monitoring of the stream effluent from watersheds and land use data.  2,760 soil samples were collected according to 5 different land use (agricultural land, forest, corral, land for house, and streamside) from four watersheds (South Han river (238,357 km²), Pyoengchang river (233,490 km²), Inbuk stream (42,674 km²), and Soyang river (102,320 km²)) located in the upstream of Hanriver basin.  Nutrients (C, N, and P), cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺), and heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) were determined for soil samples.  Stream water data (N and P concentrations and discharge volumes on biweekly basis) were obtained from 8 streaming monitoring site (IB-A, SY-A, SY-B, JC-A, PC-A, GJ-A, OD-A, and HK-A) located at the discharge point of sub-watersheds.  Flow-weighted N and P concentrations of stream discharge were calculated using an empirical model: ∑ [Nutrient-conc. (mg L^-1) × daily mean flow (m³ s^-1)] / ∑ daily mean flow (m³ s^-1).  Based on land use, N concentration was in order of agriculture land > corral > forest > land for house > streamside and P concentration was corral > agriculture land > land for house > forest > streamside showing that cultivated soil had comparatively higher N and P concentrations than the soils used for other purposes.  The levels of N and P measured at watershed effluent samples were well correlated with N and P level in soils for corresponding watershed.  Especially, the good relationship between agricultural land area normalized soil nitrogen concentration and the N load of stream was observed, suggesting stream nitrogen level being a valuable indicator of a watershed’s water quality as influenced by agricultural practice.  Further study to characterize the influence of other factors such as soil texture, slope, and rainfall intensity is on the way to give the accurate estimation of the impact of different land use on the quality of watershed effluent.