[1] We assessed the effects of historical changes in both land use and climate on the water budget of a rapidly urbanizing watershed, Ipswich River basin (IRB), in northeastern Massachusetts. Water diversions and extremely low flow during summer are major issues in the IRB. Our study centers on a detailed analysis of diversions and a combined empirical/modeling treatment of evapotranspiration (ET) response to changes in climate and land use. A detailed accounting of diversions showed that net diversions increased due to increases in water withdrawals (primarily groundwater pumping) and export of sewage. Net diversions constitute a major component of runoff (20% of streamflow). Using a combination of empirical analysis and physically based modeling, we related an increase in precipitation (2.7 mm/yr) and changes in other climate variables to an increase in ET (1.7 mm/yr). Simulations with a physically based water-balance model showed that the increase in ET could be attributed entirely to a change in climate, while the effect of land use change was negligible. The land use change effect was different from ET and runoff trends commonly associated with urbanization. We generalized these and other findings to predict future streamflow using climate change scenarios. Our study could serve as a framework for studying suburban watersheds, being the first study of a suburban watershed that addresses long-term effects of changes in both land use and climate, and accounts for diversions and other unique aspects of suburban hydrology.

The relationships of land use/land cover (LULC) on major solute concentrations in stream water were investigated for the Ipswich River basin (404 km 2 ) in northeastern Massachusetts. Stream water was sampled seven times during base flow in 43 first-order catchments and four times in 28 second-and third-order catchments. Regression analysis of the first-order catchment data indicates that NO − 3 , acid neutralizing capacity (ANC), Cl − , SO 2− 4 , and the base cations had positive, mostly exponential relationships with the increasing extent of urban + agricultural area (P < 0.05), whereas dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) had positive, exponential relationships with the increasing extent of wetland + open water (P < 0.05). Solute sources responsible for many of these relationships are human-derived constituents found in septic effluent, fertilizers, and road salts. In contrast to more conservative solutes, concentrations of NO − 3 in the first-order streams were commonly higher than in those of second-and third-order streams with similar proportions of urban + agricultural land use. Using LULC subclasses (e.g., high density residential), as well as the proportions of LULC in 50, 100, and 200 m concentric zones bordering streams, generally decreased the relationships (r 2 ) determined above. Hence, the disturbed area of the entire subbasin was the best descriptor of streamwater solute concentrations. Potassium concentrations in stream water had stronger relationships than any other ion, yet these explained <60% of the variability, indicating that there are a number of important ancillary factors that affect streamwater solute composition in the Ipswich River basin.

? uptake rate patterns had a distinct seasonal reversal; fall had the highest uptake rates in the upper reaches, while summer had the highest uptake rates in the lower reaches. This seasonal reversal was attributed to organic matter and evidenced by DON patterns. Transient storage did not have an expected effect on uptake rates in fall because it was confounded by leaf litter; litter produced higher uptakes, but also may have reduced transient storage. In summer however, uptake rates had a positive correlation with transient storage. Debris dams had no distinct effect on uptake in fall because of their recent formation. In summer however, the debris dam effect was significant; although the debris dams were hydraulically inactive then, the upstream reaches had 2-5 fold higher uptake rates. The seasonal and longitudinal differences in NH 4? uptake reflect interactions between flow conditions and the role of organic matter. Urbanization can alter both of these characteristics, hence affect stream N processing.