To determine the contribution of glacier ice and seasonal snow to runoff, we are applying a suite of satellite remote sensing, reanalysis and ground-based data as input to specific snow and ice melt models. Gridded maps of snow and glacier area/elevation are used as input to a temperature-index melt model. The melt model estimates runoff from snow covered grid cells, based on cell area and melt depth. Glacier melt is estimated in the same way, once snow has disappeared from glacierized grid cells. The melt model is driven by daily mean temperature from reanalysis data.
We are evaluating the accuracy of the melt model results using innovative isotopic and geochemical tracers to identify and quantify the sources of water (ice melt, snow melt, rainfall and ground water) flowing into selected rivers that represent the major hydro-climates of the study area. Ultimately, with our Asian partners, we will assess the performance of the various melt models. Results of this study can be applied to future efforts to assess the social-economic impacts of water uses and their vulnerability to changes in flow magnitude and timing.
Satellite Products and Processing
We use data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor to map glacierized areas and to map seasonal snow cover.
The MODIS Persistent Ice (MODICE) algorithm makes use of the time series of fractional snow and ice cover from the MODIS Snow covered Area and Grain size (MODSCAG) algorithm. MODICE produces a consistently-derived map of annual minimum exposed snow and ice. For the CHARIS project, MODICE annual maps of glacierized areas from 2000 to the present are used as input to a temperature-index melt model.
A temporally and spatially continuous daily time series of snow cover extent is being produced using MODIS MOD10A1 daily snow cover maps. Temporal and spatial persistence filters are used to fill missing and cloud covered cells in daily snow maps. Maps of snow cover extent are used as input to a temperature-index melt model.
Elevation (topographic) data are essential for modeling seasonal snow and glacier ice melt. Elevation defines the basins and water courses of a landscape and the exposure of the surface to direct and reflected solar radiation and other meteorological conditions. Accurate elevation data are difficult to obtain in the mountainous, largely inaccessible regions of this study. For this reason we must rely on digital elevation models (DEMs) generated from satellite data. The available data sources—interferometric synthetic aperture radar, satellite image stereoscopy and spaceborne laser altimetry—have shortcomings that make it necessary to combine the best features of each to obtain an optimal DEM.
The Role of Reanalysis
Near-surface and air temperature data from atmospheric reanalyses are used to drive melt and hydrological models. Rather than selecting a single best reanalysis product, we are using a suite of products, including but not limited to the three current state-of-the-art reanalyses: the NASA Modern Era Retrospective Analysis for Research and Applications (MERRA), the European Center for Medium Range Weather Forecasts (ECMWF) ERA-Interim and the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR).
River Discharge Data
River discharge data from stream gauging stations are being used for model validation and streamflow analysis. Because this study focuses on the contribution to streamflow from melting glacier ice and seasonal snow, we are using streamflow data from gauging stations that define sub-basins that include the maximum extent of seasonal snow cover and all glacier cover for each of the five major drainage basins. Additional gauging stations nested within these larger sub-basins have been selected so that contributions from highly glacierized and snow covered basins can be evaluated.
Water Sample Isotope Analysis
The CHARIS project objectives include quantifying glacier and snow-covered areas from remote sensing data and modeling the melt contribution of these hydrologic components to streamflow (hydrograph separation). In addition we are using innovative isotopic and geochemical tracers to identify and quantify the sources of water (ice melt, snow melt, rainfall and ground water) flowing into selected rivers representing the major hydro-climates of the study area. In glacier, snow and rain-fed basins, these three sources must be differentiated from each other and from the contribution of groundwater to baseflow. We are evaluating the accuracy of melt model results using these isotopic and geochemical tracers that identify and quantify water sources.