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Mean air temperature (°C)

FIGURE 2.4 Annual runoff as a percentage of precipitation versus mean annual air temperature for rivers draining into the Atlantic Ocean from Canada to the southeastern United States. (Reproduced from Allan and Benke 2005.)

2.1.3 Surface versus groundwater pathways

Precipitation destined to become runoff travels by a number of pathways that are influenced by gradient, vegetation cover, soil properties, and antecedent moisture conditions. Some rainwater evaporates from the surface of vegetation immediately during and after a rainstorm, never reaching the ground or being absorbed by plants. This is referred to as interception and is included within ET. Some rainfall passes through spaces in the canopy (throughfall), some runs down stems and trunks (stemflow), and some intercepted water later falls to the ground (canopy drip). The latter two pathways may play a role in nutrient transfers, and will be discussed later.

Once rain or melt water encounters the ground, it follows several pathways in reaching a stream channel or groundwater (Figure 2.5). Approximately three fourths of land-area precipitation infiltrates into the soil. In unsaturated,

Precipitation

Precipitation

Drakkar Normand

FIGURE 2.5 Pathways of water moving downhill. Overland flow (1) occurs when precipitation exceeds the infiltration capacity of the soil. Water that enters the soil adds to groundwater flow (2) and usually reaches streams, lakes, or the oceans. A relatively impermeable layer will cause water to move laterally through the soil (3) as shallow subsurface stormflow. Saturation of the soil can force subsurface water to rise to the surface where, along with direct precipitation, it forms saturation overland flow (4). The stippled area is relatively permeable topsoil. (Reproduced from Dunne and Leopold 1978.)

FIGURE 2.5 Pathways of water moving downhill. Overland flow (1) occurs when precipitation exceeds the infiltration capacity of the soil. Water that enters the soil adds to groundwater flow (2) and usually reaches streams, lakes, or the oceans. A relatively impermeable layer will cause water to move laterally through the soil (3) as shallow subsurface stormflow. Saturation of the soil can force subsurface water to rise to the surface where, along with direct precipitation, it forms saturation overland flow (4). The stippled area is relatively permeable topsoil. (Reproduced from Dunne and Leopold 1978.)

porous soils, water infiltrates at some maximum rate, termed the infiltration capacity. This capacity declines during a rain event, normally approaching a constant some 0.5-2 h into the storm (Free et al. 1940). The downward percolation of water results in a series of hydro-logic horizons. The unsaturated (vadose) zone lies above the saturated (groundwater, phreatic) zone whose upper limit is the water table surface. Soil moisture usually is least in the rooted zone, which is the uppermost horizon of the unsaturated zone, due to evaporation, plant uptake, and downward infiltration. The water table is the fluctuating upper boundary of the ground-water zone. These horizons fluctuate seasonally depending on rainfall, and generally rise at the end of the growing season when ET is low. Soil moisture thus varies with prior rainfall and season, and the degree of soil saturation influences whether new moisture percolates downward to recharge groundwater, moves laterally through the soil, or rises vertically above the soil surface.

Rain that reaches the groundwater will discharge to the stream slowly and over a long period of time. Base flow or dry-weather flow in a river is due to groundwater entering the stream channel from the saturated zone. Above the saturated zone, some infiltrated water will move downslope as interflow, which is subsurface runoff in response to a storm event (Figure 2.5). Interflow is lowest in unsaturated soils and when grain size (and thus pore size) is small; it can reach 11m day1 through sandy loam on a steep hill (Linsley et al. 1958). Rainfall in excess of infiltration capacity accumulates on the surface, and any surface water in excess of depression storage capacity will move as an irregular sheet of overland flow. In extreme cases, 50-100% of the rainfall can travel as overland flow (Horton 1945), attaining velocities of 10-500 m h_1. Overland flow tends to occur in semiarid to arid regions, where human activities have created impervious surfaces or compacted the soil, when the surface is frozen, and over smoother surfaces and steeper slopes (Dingman 2002).

However, overland flow rarely occurs in undisturbed humid regions because their soils have high infiltration capacities. Lastly, when there is a large rainstorm or a shallow water table, the water table may rise to the ground surface, causing subsurface water to escape from the saturated soil as saturation overland flow. This is composed of return flow forced up from the soil and direct precipitation onto the saturated soil (Dunne and Leopold 1978). Velocities are similar to the lower range of Horton overland flow.

Most rivers continue to flow during periods of little rainfall. These are perennial, as opposed to intermittent, and most of the water in the channel comes from groundwater. In humid regions the water table slopes toward the stream channel, with the consequence that groundwater discharges into the channel. Discharge from the water table into the stream accounts for base flow during periods without precipitation, and also explains why base flow increases as one proceeds downstream, even without tributary input. Such streams are called gaining or effluent (Figure 2.6a). Streams originating at high elevation sometimes flow into drier areas where the local water table is below the bottom of the stream channel. Depending upon permeability of materials underlying the streambed, the stream may lose water into the ground. This is referred to as a losing or influent stream (Figure 2.6b). The same stream can shift between gaining and losing conditions along its course due to changes in underlying lithology and local climate, or temporally due to alternation of base flow and stormflow conditions. The exchange of water between the channel and groundwater will turn out to be important to the dynamics of nutrients and the ecology of the biota that dwells within the substrate of the streambed.

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