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7. Runoff ProcessesRain and snowmelt water take various paths to streams. Each path contributes differently to;- peak and timing of storm runoff- erosion- transport of chemicals into streamsPlanners need to understand runoff processes to;- identify the area contributing to runoff generation- assess the impact of deforestation, road construction, and

other landuse change on runoff characteristics- evaluate the risk of stream pollution

Possible paths of water moving downhill: path 1 is Horton overland flow; path 2 is groundwater flow; path 3 is shallow subsurface storm flow; path 4 is saturation overland flow. The unshaded zone indicates highly permeable topsoil, and the shaded zone represents less permeable subsoil (D & L, Fig. 9-1).

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Three types of runoff processes having different characteristics (peak flow, lag to peak, chemical transport).

Horton overland flow

Infiltration capacity decreases as the soil gets wet. Overland flow occurs when rainfall intensity exceeds infiltration capacity.

Rainfall, runoff, infiltration, and surface storage (D&L, Fig. 9-4)

lag to peak0

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storm runoff

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Infiltration capacity varies considerably within a catchment depending on soil types and vegetation cover. Horton overland flow may occur in localized areas within the catchment; partial-area concept.

Horton overland flow is rare in vegetated humid region. It is common in areas devoid of vegetation such as;

- semi-arid rangelands- compacted soil, e.g. logging roads- paved urban area, e.g. parking lots

Runoff to rainfall ratio varies significantly. On catchements of less than 1km2, the ratio can be over 50 %. Peak rate of runoff generation can be as high as 100 mm/hr for these catchments. The flow velocity can reach up to 102 m/hr.

depression storage

Sheet flow occurs when depression storage is exhausted. Runoff peak is sharp and time lag is small.

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water content

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Subsurface storm flow

This graph shows water content profiles during a hypothetical storm event (100 mm of rain in 10 hours). The wetting front reaches the water table if the storm lasts long enough. As a result, the water table rises substantially.

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A hill slope with a stream at its base, the water table, and water content profiles (D & L, Fig. 9-7).

Before the onset of a storm, the water table declines gently toward the stream providing base flow (BF). During the storm, the water table near the stream rises rapidly and increases the volume of groundwater flow. This is called subsurface storm flow (SSSF).

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If a low permeability layer exists at some depth, water accumulates above this layer and flows horizontally toward the stream (Path 3 in 6-1).

Subsurface storm flow generates lower volume of runoff than Horton overland flow. Runoff to rainfall ratio is usually less than 20 %. Most of the rain is stored in the sediments and is released slowly to supply steady base flow.

Hydrograph peaks lag rainfall by a few hours to one day, even for small catchments, and the shape of the hydrograph is broader than that of Horton overland flow. The peak rate of runoff generation is usually less than 10 mm/hr for small catchments. Flow velocity of SSSF is several orders of magnitude smaller than that of Horton overland flow.

Saturation overland flow

If the rainstorm is large enough, the water table near the stream rises to the ground surface (see the bottom figure in the last page). Groundwater seeps out from the ground surface and generates overland flow. This is called return flow (RF). The rain falling on the saturated area cannot infiltrate because groundwater flow direction is upward under the area. Direct precipitation onto saturated areas(DPS) also generates overland flow. The combination of RF and DPS is called saturation overland flow.

Hydrographs of saturation overland flow have much higher peaks and shorter lag times than SSSF.

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Runoff from a steep, well-drained hillside covered with a sandy loam in Vermont (D & L, Fig. 9-9).

The peak rate of runoff generation varies, but it is less than that of Horton overland flow, because only a portion of the drainage basin is contributing saturation overland flow. Flow velocity is somewhat smaller than that of Horton overland flow, because saturation overland flow takes place on gentle vegetated surface.

This figure shows three components of storm flow. Saturation overland flow dominates in the early stage.

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Variable source concept

Before a storm, saturated areas are limited to the close vicinity of the stream. They expand during the storm resulting in larger rate of runoff generation (Fig. 9-11).

Some areas also expand and contract seasonally (Fig. 9-12). Because this runoff-producing zone occupies only a small portion of the watershed, even small changes can cause important differences in the volume and rate of storm runoff.

Map of saturated areas showing expansion during a single rainstorm of 46 mm. The solid black shows the saturated area at the beginning of rain; the shaded area is saturated by the end of the storm (D&L, Fig. 9-11).

Seasonal variation of pre-storm saturated area in a catchment in Vermont (D&L, Fig. 9-12).

7-8Runoff processes in rural areas

In arid and semi-arid regions with scarce vegetation and those disturbed by humans (urbanization, logging, etc.), infiltration capacity is a limiting factor and Horton overland flow is a dominant process. This also happens when the top soil is frozen. In most humid regions, subsurface storm flow and saturation overland flow are dominant processes.

Where the soils are well-drained, deep and very permeable, the water table is deep and the saturated zone is confined to the valley floor. Saturation overland flow is less important than subsurface storm flow in this situation.

Where the soils are thin and only moderately permeable, and slope is gentle and concave shaped, the water table is shallow and the saturated zone expand readily. Saturation overland flow dominates in this situation.

D&L, Fig. 9-16.

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Runoff processes in urban areas

Modification of the land surface during urbanization changes the type and magnitude of runoff processes.

Covering parts of the catchment with impervious roofs and concrete lots increases the volume and rate of Horton overland flow. Planners have to design detention ponds to accommodate increased runoff.

Gutters and storm sewers convey runoff rapidly to stream channels. The channels are straightened and lined with concrete to increase the efficiency, so that they transmit the flood wave downstream more quickly. A storm hydrograph after urbanization has larger peak flow and shorter lag time than before. The capacity of culverts and bridges are overtaxed and residential areas become flooded during large storms.

Effects of urbanization on storm hydrographs (D&L, Fig. 9-19).

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Overland flow and contaminant transport

(1) Experiments by Dunne and his colleagues.Fertilizers were applied on the ground surface before an

artificial rainstorm. Extent of saturated area during the storm was marked. On the saturated area, fertilizers were lost to runoff resulting in a clear boundary between good and poor vegetation growth after the storm.

(2) Snowmelt runoff in western Canada.Soil frost reduces the effective permeability, and hence

infiltration capacity. Spring runoff occurs probably as a combination of Horton and saturation overland flow. What will be the fate of manure applied on the field in the fall?

(3) Feed-lot operation in southern AlbertaA large number of live stock are fed in small feed lots

producing a large quantity of manure. Soils within feed lots are compacted and susceptible to Horton overland flow. What is the risk of bacteria and excess nutrients contaminating streams?

These problems are not well understood at the moment. There are needs for fundamental and practical research.