Runoff Processes and Their Significance

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THOMAS DUNNE

Universit!l of Washington

INTRODUCTION

Rainwater and meltwater can follow one of sever-al surface or subsurface paths down hillsides to stream channels. The migration of water along one of these paths is called a runoff process, and more than one such process usually operates at any given site. The relative magnitude of flows along each path depends on local climatic, soil, and geologic characteristics and

particu-larly on vegetation cover, as is discussed later.

Runoff processes are the central issue in land-surface hydrology because they affect the volume, rate, and timing of streamflow and therefore the generation of floods and critical-ly low dry-weather flows, the prediction of which has been the subject of much effort.

These efforts have responded uinly to design needs in water-resource development and flood control, and their aims were limited. Conse-quently, it was sufficient to ignore the complex set of processes generating streamflow and to treat the problem of runoff prediction as a statistical problem in which measured output (streamflow) was related to measured input (rainfall or snowmelt) and various drainage-basin characteristics such as area and slope.

The resulting empirical relationship could be used for predicting future streamflow in the measured drainage basin or on unmeasured catch-ments. This method has yielded useful results that formed the basis of a great deal of suc-cessful engineering. However, the statistical approach has several limitations.

1. Most hydrologic records are short and unlikely to sample extreme eventsJ the most important design events must often be estimated through extrapolation from an inadequate set of data.

2. Climatic fluctuations and human influ-ences in the hydrologic cycle constantly alter the rainfall-runoff relationship in some re-gions, so that even long records do not provide a homogeneous set of data for statistical analy-sis.

3. The statistical approach provides little or no information about the physical character-istics or spatial distribution of runoff within a drainage basin. Recent interest in the scien-tific aspects of hydrology and in the applica-tion of hydrologic studies to a wider range of societal problems have focused attention on these runoff characteristics.

Runoff from hillsides entrains and

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ports into channels the sediment, chemical, organic debris, bacteria, viruses, and other materials that affect the characteristics of streamwater. On its way to the channel, runoff is associated with various degrees of soil wet-ness and pore pressure, which affect such impor-tant land characteristics as suitability for agriculture or waste disposal, trafficability, and stability against landsliding.

Human activity frequently alters the inten-sity of runoff mechanisms or introduces a pro-cess that did not occur in the area before disturbance. Thus, changes in land use are often accompanied by increases in the size of floods on small streams, waterlogging of soils, and landsliding or high concentrations of sedi-ment, chemicals, or biological pollutants in streamwater. The disturbances spread and their effects accumulate while subject to a random array of meteorological events. Consequently, it is exceedingly difficult to predict the in-fluences of human activity on the basis of empiricism alone, because it is not possible to compile adequate data sets through repetition of controlled experiments. Various federal agencies have expended a great deal of effort on such experiments in small drainage basins, but the statistical limitations of the experi-mental designs and the inherent difficulty of

interpreting spatially distributed processes on the basis of measurements at the basin outlet alone severely restrict the conclusions that can be drawn from such experiments. Thus, after 30 yr of such research there is still controver-sy and confusion about such issues as the influ-ence of forest management on flood peaks, soil nutrient status, sediment production, and water

quality. ·

Many questions concerning land and water management are likely to be of this type in the next few decades. For example, the problem of predicting nonpoint sources of phosphorus, nitrogen, or bacteria in streams and lakes or of the wash-off of heavy metals and other pol-lutants in urban storm drainage requires that the amount and path of runoff and its hydraulic characteristics can be predicted. A statistical relationship that would predict peak runoff is not adequate. The sources and characteristics of the runoff and what it can entrain and some effects of large-scale exploitation of tropical landscapes, such as the Amazon Basin.

Concerns such as the potential for physical and chemical degradation, the alteration of biogeo-chemical cycles, and the ·stream transport of organic materials involve runoff. Yet, only one small experimental investigation of runoff processes has been conducted in the 6.1-million making predictions about the hydrologic conse-quences of disturbanceJ about the probable dis-tribution of accelerated erosionJ or of leach-ing, waterloggleach-ing, or any other impact.

In this and many other situations the conse-quences are likely to be predicted on the basis of simple statistical relationships that incor-porate coefficients to predict volumes or peak rates of runoff, amounts of soil loss, or other requirements. These conceptually simple rela-tionships are often applied to small portions of drainage basins and added together or other-wise manipulated by high-speed computers and presented as complex models. Close examination of such calculations frequently indicates that the application is fundamentally wrong. The relationships are often developed with the wrong runoff process or erosion and transport mecha-nism in mind, or the parameters are unreasonable in light of field observations or physical pos-sibilities. Despite the fact that such calcula-tions may not describe what is happening in the our prediction personnel in field observation or data collection.•

If prediction efforts continue along these lines they will lead to expensive failures and mismanagement of land and water resources.

Errors are most likely to appear in the predic-tion of extreme events or disruppredic-tions of kinds that have not been well documented. There will also be a lack of intelligent approaches to problems other than prediction of some stream-water characteristic at the basin outlet. In ap-proach to hydrologic prediction has emerged

through a combination of field experiments and mathematical models. These developments concen-trate on the physics of runoff and of the physically based modeling capability.

DESCRIPTION OF RUNOFF PROCESSES Horton Overland Flow

The paths that water can follow to a stream are indicated schematically in Figure 1.1. An

im-Hodels of Runoff Processes and Their Significance portant separation occurs at the soil surface, which can absorb water at a certain maximum rate known as the infiltration capacity. This rate is relatively high at the onset of rainfall but declines rapidly to an approximately constant value after about 0.5-2 h. If rainfall intensi-ty at any tiM during a rainstorm exceeds the infiltration capacity of the soil, water accu.u-lates on the surface, fills small depressions, infiltration and runoff and their consequences for land and water management and who also con-ducted soM of the earliest experiments on these processes. The occurrence of Borton overland flow depends mainly on the surface characteris-tics that control infiltration, the most impor-tant of which are vegetation and soil condi-tions. Dense vegetation cover protects the soil surface from the packing effect of raindrops, provides organic material, and promotes biolog-ical activity that forms open stable soil aggre-gates. Coarse-textured soils can absorb rain-fall at a higher rate than silty or clayey soils. Land use often involves the reduction of vegetation cover and the breakdown of soil aggregates, with a consequent lowering of infil-tration capacity and an increase in the humid landscapes, such as cultivated fields, paved areas, mine spoils, construction sites, and rural roads. These areas lack a dense vege-tation cover and well-aggregated topsoil. In areas where Borton overland flow is the dominant prediction is the determination of whether over-land flow will occur on a certain portion of the

Betson {1964) referred to the nonuniform genera-tion of Borton overland flow as •partial-area runoff.•

Horton overland flow typically travels over hillslope surfaces at velocities of 10-500 m/h, runoff processes. Prediction of response time is another important goal of runoff modeling. permeable, the infiltrated water percolates through the soil into the vadose zone {see important con-tributor to storm runoff. {The characteristics, significance, and modeling of this runoff process are examined in Chapters 4 and 5.)

In many soils or rocks, water percolating vertically encounters an impeding horizon and is diverted laterally. It migrates downslope through the soil and displaces into the stream channel water that is in storage and has been migrating slowly downslope during the preceding days or weeks. If this shallow subsurface flow (path 3 in Figure 1.1) is generated under a low rainfall intensity onto a highly permeable soil, the water may travel downslope without saturat-ing the soil. Flow is confined to the narrower, intergranular pores, and its velocity is usually of the order of 10-4 11/h or less. Under in-tense rainstorms the soil often becoaes saturat-ed at some depth. Water is then able to migrate through the largest pores, rootholes, wormholes, and structural openings, and its velocity may

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shallow subsurface flow declines and may cease altogether in some soils. Hewlett (l96la, l96lb) showed, however, that in a mountainous region underlain by impervious rock, slow drain-age of water from unsaturated soils may be responsible for dry-weather flow also. Recent important work has been reported by Barr (1977), Anderson and Burt (1977, 1978), and Anderson and Kneale (1980) •

Saturation Overland Flow

Vertical and horizontal percolation may cause the soil to become saturated throughout its depth on some parts of a hillside. The soil saturates upward from some restricting layer, and when saturation reaches the ground surface, rainfall cannot infiltrate but runs over the surface as saturation overland flow. Some of

SCALE

0 1000 2000 3000 4000 Ft.

Contour lntervoiiOO Ft.

/ April 26, 191'~

/ (A1•12%)

a

TBCIW3 DTJNNE the water moving slowly through the topsoil aay emerge and flow overland to the channel as re-turn flow (Musgrave and Holtan, 1964). Thus, direct precipitation onto the saturated soil, with or without return flow, generates satura-tion overland flow (path 4 in Figure l.l), which is intimately related to subsurface runoff and soil conditions. The process occurs most frequently on gentle footslopes and hollows with shallow, wet soils (Dunne, 1970) but can also occur on other parts of a hillslope with thin or wet soils. During a storm the saturated zone may spread upslope and especially into topo-graphic hollows and zones of shallow, wet, or less permeable soil. Slower fluctuations of the zone producing overland flow result from season-al changes in wetness. Figure 1.2 provides field examples of such fluctuations. Saturation overland flow usually moves more slowly (0.3 m/h to less than 100 m/h) than Horton overland flow because it travels through dense ground vegeta-tion on low gradients. It generates streamflow with a lag time and peak discharge that are intermediate between those characteristic of Horton overland flow and of subsurface storm-flow.

It is now generally agreed that in densely vegetated humid regions, most storm runoff is generated by a combination of shallow-subsurface stormflow and saturation overland flow, which in turn consists of various proportions of

NORTH CONTOt.ft INTERVAL 10 FEET

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FIGURE 1.2 (a) Seasonal variation of the saturated zone in a catchment at Randboro, Quebec. (b) Expansion of the saturated area during a 46-mm rainstorm in a catchment in northern vermont. The solid area indicates the sat-urated zone at the beginning of the storm1 the lined area indicates that zone at the end of the storm (from Dunne et al., 1975).

Hodels of Runoff Processes and Their Significance Direct precipitation and return flow /

dominate hydrogroph;

subsurface stormflow /

less important

+