Kirsten M. Menking

Contributions of La niña and El Niño to middle Holocene drought and late Holocene moisture in the American Southwest

Eolian landforms in Estancia basin, central New Mexico, record two episodes of extreme drought and low groundwater levels during the middle Holocene (7000-5400 1 4 C yr B.P.), followed by a rise in the water table throughthe late Holocene. Blowouts and associated lunettes formed when water levels fell below the desiccated floor of pluvial Lake Estancia, allowing widespread deflation. Elevation of the water table in the basin is regulated by a balance between evaporation from playas that occupy the blowouts and recharge of aquifers in the adjacent Manzano Mountains. Isotopic analyses of modern precipitation and groundwater reveal that recharge originates primarily as winter moisture, which is amplified during El Ninos and diminished during La Ninas. Thus, changes in the elevation of a reconstructed Holocene water table may reflect systematic changes in El Nino-Southern Oscillation (ENSO) circulation.

THE SUBURBAN STREAM SYNDROME: EVALUATING LAND USE AND STREAM IMPAIRMENTS IN THE SUBURBS

Development is known to impair stream water quality at moderate to high levels of urbanization, but the effects of low-density urban expansion, the kind occurring on the outskirts of many cities, remain unclear. We examined five suburban headwater streams in Duchess County, New York whose watersheds contained between 4.7% and 34% impervious surface cover. We measured Cl- and nitrate-N (NO3-N) concentrations in water samples taken at four to six sites on each stream in winter and summer. Even at low levels of population and impervious cover, concentrations of both Cl- and NO3-N exceeded reference levels found in cleaner streams in the region. Chloride levels were elevated in upper reaches and remained elevated or continued to increase downstream, with a linear response to impervious cover. Nitrate-N increased downstream in all watersheds, indicating that NO3-N inputs exceeded natural denitrification and uptake in both winter and summer. Nitrate-N responded logarithmically to impervious surface cover, with steep increases at low levels of imperviousness. Per-capita inputs were also high in rural areas. Agricultural inputs were not sufficient to explain observed trends in NO3-N; we interpret inputs to result chiefly from low-density exurban expansion. Widespread residential expansion has significant impacts on water quality that have not previously been acknowledged.

Model estimates of runoff in the closed, semiarid Estancia basin, central New Mexico, USA

Abstract The 5000 km2 topographically closed Estancia basin in central New Mexico has been the focus of several palaeoclimatic studies based on changes in the level of late Pleistocene Lake Estancia. A large, unknown volume of surface runoff and groundwater from adjacent mountains contributed to the hydrological balance during highstands and lowstands. The US Department of Agriculture hydrological model SWAT (Soil and Water Assessment Tool) and the US Geological Survey groundwater flow model MODFLOW, with the LAK2 package, were used in this study to estimate runoff and water balance under present climate. A Geographic Information Systems (GIS) interface was used for SWAT, digitized data were applied for soils and vegetation, and limited streamflow data were used to obtain an approximate calibration for the model. Simulated streamflow is generally within 30% of observed values, and simulated runoff for the entire basin is about 8% of the annual inflow volume needed to support lowstands of the former Lake Estancia. Results from the combined models suggest application to other palaeoclimate investigations in semiarid lake basins.

INFLUENCE OF OPEN SPACE ON WATER QUALITY IN AN URBAN STREAM

Much attention has been given to the impairment of streams in urban areas and to the value of green space in preventing degradation. However, few studies have examined whether green space can remediate water quality downstream of contaminant sources. To assess the degree to which an ecological preserve was able to ameliorate upstream water quality impairments, we examined changes in conductivity, total inorganic nitrogen (TIN), and a family biotic index (FBI) for benthic macroinvertebrates in a partially urbanized stream in eastern New York state, USA. We expected conductivity, which results mainly from road de-icing salt, to decrease in the green space due to dilution from low-conductivity surface runoff. We also expected TIN and FBI to indicate stream improvements in response to increased vegetative cover in the green space. Contrary to expectations, conductivity did not improve in the ecological preserve, although TIN and FBI values did improve. Differences in scales of response explain this contrast in recovery/conductivity responded to basin-wide percentage impervious surface cover (ISC), while TIN and FBI responded to riparian-scale ISC, which declined sharply in the ecological preserve. Conserving riparian green space can aid natural recovery of TIN and FBI. In contrast, controlling conductivity requires watershed-wide management.

Evaporation from groundwater discharge playas, Estancia Basin, central New Mexico.

Abstract Bowen ratio meteorological stations have been deployed to measure rates of evaporation from groundwater discharge playas and from an adjacent vegetated bench in the Estancia Basin, in central New Mexico. The playas are remnants of late Pleistocene pluvial Lake Estancia and are discharge areas for groundwater originating as precipitation in the adjacent Manzano Mts. They also accumulate water during local precipitation events. Evaporation is calculated from measured values of net radiation, soil heat flux, atmospheric temperature, and relative humidity. Evaporation rates are strongly dependent on the presence or absence of standing water in the playas, with rates increasing more than 600% after individual rainstorms. Evaporation at site E-12, in the southeastern part of the playa complex, measured 74 cm over a yearlong period from mid-1997 through mid-1998. This value compares favorably to earlier estimates from northern Estancia playas, but is nearly three times greater than evaporation at a similar playa in western Utah. Differences in geographical position, salt crust composition, and physical properties may explain some of the difference in evaporation rates in these two geographic regions.

Using Mobile Mapping to Determine Rates of Meander Migration in an Undergraduate Geomorphology Course

Students in an undergraduate geomorphology course used tablet computers equipped with Global Positioning System (GPS) receivers and Geographic Information Systems (GIS) software to map the current location of a stream that crosses the Vassar College campus in Dutchess County, New York. They also digitized the position of the stream on aerial photographs taken in 1959 to determine meander migration rates over the last ~45 years of ~0.2 m/yr. The purpose of the exercise was to introduce basic GIS skills, such as georeferencing of air photos, digitization, acquisition of GPS data points, data projection, and creation of maps. Students were evaluated on the quality of their maps and on an accompanying short scientific article. Here we discuss the exercise and the pros and cons of the tablet PC technology.