Summary

Fluvial ecosystems exhibit tremendous variability in the quantity, timing, and temporal patterns of river flow, and this profoundly influences their physical, chemical, and biological condition. Vast quantities of fresh water are extracted to meet agricultural, municipal, and industrial demands, yet freshwater ecosystems also need enough water, of sufficient quality and at the right time, to remain ecologically intact and provide economically valuable commodities and services to society. Increasingly today, the tools of hydrologic analysis are being combined with other elements of river science to address the

FIGURE 2.17 Natural (light shading) and modified (dark shading) hydrographs for selected rivers in South Africa, illustrating a flow scenario that provides only half as much water as the natural hydrograph and is intended to accommodate human uses while keeping the river as healthy as possible. This scenario illustrates some key principles identified by a panel of scientists. (Reproduced from Postel and Richter 2003, after Tharme and King 1998.)

FIGURE 2.17 Natural (light shading) and modified (dark shading) hydrographs for selected rivers in South Africa, illustrating a flow scenario that provides only half as much water as the natural hydrograph and is intended to accommodate human uses while keeping the river as healthy as possible. This scenario illustrates some key principles identified by a panel of scientists. (Reproduced from Postel and Richter 2003, after Tharme and King 1998.)

question, ''How much water does a river need?'' and ensure that flows are sufficient to protect and restore stream ecosystems.

The hydrologic cycle describes the continuous cycling of water from atmosphere to earth and oceans, and back to the atmosphere. Evaporation from the oceans exceeds that over land, while precipitation on the earth's land surfaces exceeds evaporation and plant water loss. This excess provides the runoff from land to sea that is primarily river discharge, but includes groundwater as well. At the scale of an individual catchment and averaged over years, a water budget consists of inputs from precipitation and groundwater inflow, and outputs due to streamflow, ET (water loss from evaporation and by plants), and groundwater outflow. Over short time intervals, imbalances between inputs and outflows result in increases or decreases in storage. Globally, almost two thirds of precipitation that falls on land becomes ET, and ET exceeds runoff for most rivers and for all continents except Antarctica. Both precipitation and ET vary with climate and vegetation, resulting in seasonal and regional differences in the amount and timing of streamflow.

Precipitation destined to become runoff travels by a number of pathways that are influenced by gradient, vegetation cover, soil properties, and antecedent moisture conditions. Surface and shallow subsurface flows reach streams much more quickly than water that percolates to the water table and discharges into the stream as ground-water. Thus the stream hydrograph, which describes the rise and fall of streamflow over time, will exhibit a strong or a more gradual response to a rain event depending on soils, slopes, and human actions that affect flow paths. Most rivers continue to flow during periods of little rainfall, and this base flow comes from groundwater that discharges into the channel more or less continuously, depending on fluctuation in the level of the water table.

The characterization of streamflow has practical application for the design of flood-control structures, evaluation of channel stability, and in determining whether sufficient water is available at the appropriate time to meet the needs of both people and the ecosystem. One can estimate the frequency with which flows of a given magnitude are expected to occur, or the probability of occurrence of a flow of a given magnitude. The flood magnitude that has a 1- to 2-year recurrence is often used as an estimate of the flood that just overtops the banks, which is the flow considered most influential in maintaining channel form.

Flow analyses tell us that each individual river has a natural flow regime characterized by the magnitude of flows and their frequencies, as well as duration, timing, and rate of change. Climate, vegetation, geology, and terrain place broad constraints on natural flow regime, conditions at the catchment scale make each river to some degree unique, and a wide range of human influences further alter flow regimes by changing flow pathways and response times, and even by altering climate. In highly regulated rivers, once-seasonal flows have become nearly constant due to impoundments, whereas other rivers have become much flashier owing to changes in stormwater conveyance and impervious surfaces. The science of environmental flow assessment has developed in response to the recognition that river flows are being altered everywhere, causing widespread ecological degradation. Present evidence suggests that rivers and their floodplains need much of the spatial and temporal variability of their natural flow regime to maintain their ecological integrity. Our ability to prescribe environmental flows that protect the fluvial ecosystem and meet human needs is in its infancy, but promises to make important contributions to river health.

Chapter three

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