M. L. Shelton

DECADAL CHANGES OF INFLOW TO THE SACRAMENTO-SAN JOAQUIN DELTA, CALIFORNIA

The Sacramento-San Joaquin Delta is both an important environmental resource and a critical link in the water supply system for California. Concern for the adequacy of Delta water supplies increases with growing population and environmental maintenance needs and with the hydroclimatic uncertainty of global warming. Reconstructed or unimpaired discharges for Delta tributary areas are analyzed for trend and for changes in the seasonal regime of Delta inflows. Nonparametric tests indicate the absence of trend for annual inflows, but the low inflow months of September and October display increasing trends that are statistically significant. Additional changes in the Delta inflow regime are evident when inflow volumes are expressed relative to annual inflow. Decreasing trends in the spring fraction of annual total inflows and in the monthly fractions for April and May are statistically significant. March displays a significant increasing trend in the monthly fraction of inflow. The emerging decadal changes in ...

Irrigation induced change in vegetation and evapotranspiration in the Central Valley of California

Landscape changes in the Central Valley of California, USA, have been dramatic over the past 100 years. Irrigated agriculture has replaced natural communities of California prairie, riparian forest, tule marsh, valley oak savannah, and San Joaquin saltbrush. This paper addresses the implication of vegetation change on evapotranspiration as a consequence of these changes. It was found that an increase in irrigated agriculture and a 60% reduction in the aerial extent of native vegetation has not produced significant changes in the moisture transfer to the atmosphere. The apparent reason for this result is that irrigated agriculture has substituted one actively transpiring surface for another and, therefore, has not significantly altered the transpiration flux of the landscape.

SEASONAL HYDROCLIMATE CHANGE IN THE SACRAMENTO RIVER BASIN, CALIFORNIA

Hydroclimate change that alters runoff seasonality is an increasing concern in snowmelt-influenced watersheds throughout the western United States. Detecting seasonal runoff change is complicated by the interaction of atmospheric and topographic influences and by human alteration of streamflow. Reconstructed or unimpaired discharges for 1921 to 1994 for the Sacramento River are employed as an indicator of natural discharge and are analyzed for evidence of seasonal change. Rescaled adjusted partial sums (RAPS) improve visualization of trends and fluctuations in annual discharge and the seasonal fractions of annual discharge. The statistical significance of trends indicated by the RAPS is evaluated using nonparametric tests. Small increasing trends beginning in the late 1940s in annual unimpaired discharges and the seasonal fractions for fall, winter, and summer are not statistically significant. In contrast, the spring fraction of annual discharges shows a decreasing trend beginning in the late 1940s that ...

SPATIAL SCALE INFLUENCES ON MODELED RUNOFF FOR LARGE WATERSHEDS

Modeling the runoff subsystem for large drainage basins requires simplification in representing natural processes owing to the spatial variability of mass and energy transfers in a watershed. Many models represent temporally and spatially distributed watershed variables by some form of aggregation or lumping. Scale becomes a factor in the lumping decision because some of the variables are spatially continuous while others are spatially discontinuous. The effects of scale differences on lumping in large watersheds is examined using the 27,300 km2 drainage area of the Deschutes River, Oregon. The watershed is modeled using three different spatial aggregations for representing the runoff subsystem. Agreement between modeled and observed monthly runoff for water years 1951-60 is analyzed to evaluate the magnitude of the difference between corresponding observed and modeled values. Increasing the number of spatial units in the model from 1 to 9 reduces all of the error terms by about 35 percent, but 20 spatial...

Natural and Human Factors in Recent Central Valley Floods

Widespread and destructive flooding occurred in 1986 and 1997 along rivers flowing into California’s Central Valley. The region is historically flood prone, but extensive flood control measures are designed to minimize damage. In recent years, land use changes and population increases have placed more people and property at risk, and the underperformance of flood control facilities exacerbate flood conditions. In 1986 and 1997, powerful subtropical storms delivered heavy rainfall over watersheds draining into the Central Valley and produced contrasting flood patterns Precipitation occurrence and intensity and snowpack melting influenced peak discharge levels, but water management and land use decisions contributed to flood losses. Flooding in 1986 was most extensive in the Sacramento River Basin, where dams, levees, weirs, and other flood control structures were only partially successful in preventing flooding. The 1997 floods were most severe in the San Joaquin River Basin, where levee failures reduced river stages downstream but allowed flood waters to inundate Valley areas seldom associated with recent flooding problems. Despite early concerns, the timing and spatial characteristics of the 1998 ENSO-related precipitation produced only modest flooding in the Central Valley.

CLIMATE CHANGE AND FUTURE HYDROCLIMATE FOR THE UPPER CROOKED RIVER, OREGON

Upper Crooked River drains a semiarid elevated lava plain supporting a mixed vegetation cover of juniper, sagebrush, and grass. The majority of basin farm income is derived from livestock grazing on non-irrigated public and private land. Average annual water balance surplus is a modest 75 mm. A watershed model run for 108 consecutive months defines present-day hydroclimate as a basis for assessing changes related to global warming. Future watershed surplus is modeled assuming current land-use and grazing practices using monthly temperature and precipitation changes derived from a limited area model nested in a global circulation model. In the simulated warmer and wetter climate, peak surplus occurs three months earlier in January as an increased proportion of precipitation occurs as rain rather than snow. Severe decreases in water availability to plants accompany elevated evapotranspiration in all months. The sensitivity of the watershed to evapotranspiration is attributable to the cool season concentrati...

HYDROCLIMATIC ANALYSIS OF SEVERE DROUGHT IN THE SACRAMENTO RIVER BASIN, CALIFORNIA

Climatic water budget principles provide an analytical basis for expressing drought severity as the cumulative moisture deficiency in biologic and hydrologic systems. This conceptual framework emphasizes the disparity between real-time and expected precipitation, soil moisture, and runoff in quantifying the cumulative moisture deficiency, and it facilitates incorporation of spatial characteristics in assessing drought severity. Comparison of the temporal and spatial properties of the severe 1924 and 1977 droughts in the Sacramento River Basin illustrate the procedure. The 1924 drought has a larger average water deficiency, or magnitude, but the 1977 drought has a longer duration and its cumulative moisture deficiency, or severity, is 1.4 times greater than the 1924 drought. Expressed volumetrically, the greater cumulative moisture deficiency for the 1977 drought is 13.3 times larger than the moisture deficit difference indicated by calculations of natural runoff for water years 1924 and 1977.

Water Supply and Climate Change in the Upper Deschutes Basin, Oregon

Climate change is expected to alter the time and space characteristics of the global hydrologic cycle and to impact regional water supplies. The Upper Deschutes Basin is in one of Oregon’s fastest growing regions, and the increasing population is straining regional water resources. Surface water is fully allocated and increased groundwater use will require careful management to offset seasonal or long-term declines in aquifers or the depletion of stream flow. While altered temperature and precipitation accompanying global change are both concerns, the watershed is more sensitive to changes in precipitation than in temperature. Watershed climate simulation reveals a 25 percent increase in mean monthly runoff, and extremely high monthly runoff is four times more frequent. These changes indicate an increased risk of winter floods, greater spring and summer runoff, and a shift in the occurrence of the minimum runoff month to earlier in the year. Increased potential evapotranspiration, a decrease in the amount of precipitation stored as snow, and changes in the amount and timing of runoff will constrain water development options for humans, agriculture, and regional fisheries. Water restrictions will magnify water-use conflicts in the watershed and increase the risk of regional economic discord.

CLIMATE CHANGE AND ALTERED RUNOFF IN THE SEMIARID PORTION OF MONO BASIN, CALIFORNIA

Mono Basin is a closed hydrologic unit and is highly sensitive to anticipated changes in temperature and precipitation related to global warming. Available water in the basin is limited and intensely managed. Cottonwood Creek and Bridgeport Creek drain the northern non-Sierra portion of the Mono Basin, and while these watersheds contribute relatively little of the total runoff into Mono Lake, their hydroclimatic conditions are representative of the majority of the Mono Basin drainage area. The hydrologic balance for these watersheds is modeled and selected climate change scenarios are simulated to determine the magnitude of change in water surplus resulting from altered temperature of ±1° and ±2° C and precipitation changes of ±10% and ±20%. Changes in modeled water surplus expressed as a percentage of existing conditions range between 154% and 35% for Cottonwood Creek and 174% and 20% for Bridgeport Creek. Both watersheds display a greater response to a given change in precipitation than to a change in t...

TIMBER HARVESTING AND THE HYDROLOGIC RESPONSE OF REDWOOD CREEK, CALIFORNIA

Timber removal on publicand private land surrounding Redwood National Park exacerbates naturally high rates of erosion that are common to this region and alters hydrologic processes within the park boundaries. These alterations of the natural environment complicate the efforts of the National Park Service to preserve a remnant of the once extensive coastal redwood ecosystem in the park. A watershed model for Redwood Creek calibrated to pre-logging conditions is employed to define and quantify changes in the hydrologic response of the basin during the years when timber harvesting reduced significantly the acreage of redwoods. Analysis of modeled and observed runoff indicates that timber removal is related to increased runoff during wet months and wet years, but runoff is reduced during dry months and dry years. These alterations in the hydrologic system occur at the least beneficial time because they augment high flows, whereas low flows are depleted. Such changes in runoff contribute to magnified erosion ...