Hydroclimatic Parameters Research Articles

CHANGING TEMPORAL AND SPATIAL CHARACTERISTICS OF MIDWESTERN HYDROLOGIC DROUGHTS

Hydroclimatic parameters were examined to identify spatial variability among 19 Midwestern basins during the 1950–1990 period. The magnitude of annual streamflow variability values increased westward across the Midwest and appeared to be associated with a similar pattern in annual precipitation variability. Basin seasonal streamflow values were analyzed to determine the occurrence of hydrologic drought. A criterion that combined duration and severity of low-flow characteristics was used to determine hydrologic droughts. Sixteen of the 19 basins experienced between one and four hydrologic droughts, whereas 3 experienced none. The temporal distribution and spatial extent of the 45 hydrologic droughts indicated that there were two classifications of Midwestern hydrologic drought during this period. The first type–“long-term” hydrologic drought–occurred over: (1) a period of 12 or more consecutive seasons and (2) a broad latitudinal belt across the Midwest. Long-term hydrologic droughts occurred during the pe...

Groundwater circulation in a closed desert basin: Topographic scaling and climatic forcing

The circulation of groundwater and solutes in a closed basin in western Utah (Pilot Valley) is studied to gain insight into the climatic processes driving flow in this desert environment. The study area is located within the Great Basin portion of the Basin and Range Physiographic Province, a region which is hydrologically closed with no outlet except to the atmosphere. The distinctive topography of steep mountains adjacent to flat playas has a major influence on the climatic and hydrologic patterns of the region. A scaling relationship is found for the distribution of elevation in Pilot Valley, suggesting that the mountain slopes are self‐similar. This relationship is used to approximate the spatial distribution of orographic precipitation and recharge for groundwater model input. On the playa, or discharge zone, high evaporation rates produce concentrated brine solutions which lead to large horizontal and vertical density gradients in underlying groundwater. Numerical experiments demonstrate the existence of a free convection cell and recirculation of solutes under the playa and towards the basin margin. It is shown that the motion of the salt nose which forms is a result of a balance between the buoyancy velocity of the evaporating brines on the playa and recharge from the mountain range. A Rayleigh number for solutes is defined for basin scale hydroclimatic parameters. The simulated flow pattern is consistent with field observations in Pilot Valley and elsewhere, in that both fresh and brackish springs emerge along the margin of the playa. The concept of recirculation may also account for the apparent “loss” of salts in closed lakes as a subsurface storage mechanism.