A study of streams draining agricultural and forested land showed that the agricultural catchments had higher mean concentrations of base cations (Na, Mg, K), bicarbonate, acid anions (SO₄, Cl), metals (Al, Fe) and nutrients (NO₃, PO₄). Increased base cations apparently result from tillage practices exposing fresh soil material to weathering. Increased acid anions result from inputs of inorganic fertilizer, notably ammonium sulphate, and from an apparent increase in evapotranspiration from the flooded terraces in the agricultural catchments. Increased metal concentrations may be promoted by increased weathering and erosion rates, and this is supported by the higher turbidity in the streamwater draining the agricultural catchments. Higher levels of nutrients are the direct result of fertilizer input but concentrations are generally low from all catchments as a result of denitrification, indicating that eutrophication downstream is not a likely consequence of land use change. The major dynamics of water chemistry occur during the monsoon, which is also the main season for agricultural production. Mean wet season concentrations of base cations tend to be lower than in the dry season at all catchments as higher flow dilutes the relatively constant weathering input. Ammonium concentrations are higher from the agricultural catchments in the wet season as a result of direct washout of fertilizer. Detailed monitoring through storm periods at one agricultural catchment indicates that the chemistry responds very rapidly to changing flow, with cations decreasing and acid anions increasing followed by equally rapid recovery as flow recedes. Bicarbonate concentrations also decline markedly but are still sufficiently high to maintain pH near neutral throughout the storm event. The impacts of agricultural land use on streamwater chemistry are unlikely to lead to potentially damaging consequences for the aquatic biota. The potential for acidity generation as a result of the high applications of nitrogen fertilizers is apparently buffered by the tillage practices, which promote higher weathering and higher concentrations of base cations.