Plum Island Ecosystem LTER

The Effect of Land-use and Climate Changes on the Water Budget of the Ipswich River Basin

Human activities have altered enormously the timing, magnitude and nature of inputs of materials such as water, sediments, nutrients and organic matter to estuaries (Hopkinson and Vallino 1995). Climate variability and long-term patterns of climate change also have immense effects on the timing, magnitude and nature of material inputs. These inputs largely dictate estuarine water residence time, the distribution of salinity and habitats, sedimentation and marsh accretion as well as trophic status. One of the goals of the PIE LTER is to document climate and land use change historically and to examine the effect of these changes on material inputs.

Figure 1.

Our focus is on the hydrologic cycle of the Ipswich River watershed, the largest watershed draining into Plum Island Sound. The Ipswich River basin has undergone major changes in land use during the past 300 years, but especially in 20th century. During colonial times and through the middle of the 19th century, forestland was cleared for agricultural use. Agricultural abandonment and forest re-growth ocurred from around 1850 and continued through the first half of the 20th century. Beginning in the 1950s, urbanization increased dramatically (Fig 1). As a result, considerable water is exported from the Ipswich river Basin. Export for public water consumption outside the basin has not changed much overtime, but the gradual increase in the export of sewered wastewater has become an important component of overall diversions. Overall net diversions constitute about 10% of total precipitation or about 20% of streamflow.

The historical water budget indicates that precipitation has increased at an average rate of 2.9 mm per year (Fig 2). Runoff (streamflow + diversions) does not display a significant long-term trend. Evapotranspiration, calculated by difference from precipitation and runoff, displays a significant trend (1.6 mm per year), which is supported by independent climatological evapotranspiration estimates from ET models.

Figure 2.

We are interested in how the hydrologic budget will change in future years, given the expected increase change in global climate. We are also interested in how continued urbanization will affect the timing and magnitude of not only water but also nutrients and sediments. A combination of long-term observations and modeling under support from the LTER program will enable these types of analyses to be conducted.

The Ipswich river typically dries out during the summer, due to high rates of evapotranspiration, and because communities outside the watershed pump drinking water from the river, diverting it to other basins. This drying out results in loss of habitat for plants and animals that depend on the river, as well as decreased water quality in the reaches that do persist. To the extent that ET increases, this problem will only worsen.

The results of long-term research in the Ipswich Basin are especially important for management, especially when considering the quantity and timing of groundwater pumping for residential use. If the current trends of evapotransipiration and climate change continue, the water problems will worsen and residents will be allotted less and less water, forcing water or recycling or restrictions on use