Training Course in Watershed Management

2. The Hydrologic Cycle

Managers of watershed lands must address specific questions in relation to land use. These questions include:

• What land use activities can take place without causing undesirable hydrologic effects, such as flash flooding, erosion and sedimentation, and water quality degradation?
• What land use alternatives might be implemented to change a hydrologic response for a beneficial purpose, such as increasing water yield, improving water quality by reducing sedimentation, and reducing flood damages?

To answer questions such as these, relationships between watershed management practices and resulting hydrologic responses can be analyzed by studying the hydrologic cycle (Figure 1). Importantly, the hydrologic processes of the biosphere, and the effects of vegetation and soil on these processes, must be understood. The hydrologic cycle is complex, but it can be simplified as a series of storage and flow components.

Permission to use illustration from Physics 161 Online granted by Greg Bothun, University of Oregon

Figure 1. The hydrologic cycle consists of a system of water-storage compartments, and solid, liquid, or gaseous flows of water within and between the storage points.

Water Budget Concept

The water budget is a concept in which components of the hydrologic cycle are categorized into input, output, and storage components. To illustrate this concept for a watershed and specified time interval:

I - O = S

The water budget represents an application of the "conservation of mass" principle to the hydrologic cycle. It is essentially an accounting procedure that quantifies and balances these hydrologic components for a watershed. Coupled with energy, precipitation is the primary input to a watershed system. A portion of this precipitation input is intercepted and evaporated, which represents a loss from the soil-moisture reserve or the water-flow process. Infiltration is the process of water entering the soil surface. Evapotranspiration, which represents the sum of all of the water evaporated and transpired from a watershed, is the most difficult of all of the components of a water budget to quantify. However, the evapotranspiration component and its linkage to soil water storage and the movement of water off of a watershed is one of the hydrologic processes most affected by vegetative manipulations. Relationships of precipitation, infiltration, and soil water storage affect volumes and rates of water movement downstream.

That part of the precipitation input that runs off a land surface and the part that drains from the soil and, as a consequence, is not consumed through evapotranspiration is the water-flow component of the hydrologic cycle. Some water flows quickly to produce streamflow, while other water flows (for example, the water that flows through groundwater aquifers) can take weeks or months to become streamflow. The streamflow response of a watershed is the integrated response of the various pathways by which "excess precipitation" moves.

The most direct pathway from precipitation to streamflow is that part of the precipitation that falls into stream channels, called channel interception. Channel interception causes the initial rise in a streamflow hydrography after which the hydrograph recedes soon after the precipitation stops. Surface runoff, also referred to as overland flow, occurs from impervious areas or areas on which the rate of precipitation exceeds the infiltration capacity of the soil. Some of the surface runoff is detained by the roughness of the soil surface, but, nevertheless, it represents a quick flow response to a precipitation input. Subsurface flow, also called interflow, is that part of the precipitation that infiltrates the soil, but it arrives in the stream channel over a short enough time period to be considered part of the stormflow hydrograph.

Watersheds in dryland environments frequently exhibit lower infiltration capacities and shallow soils with lower soil moisture storage capacities in contrast to watersheds in more humid regions. Surface runoff, therefore, is an important pathway of flow from these watershed lands. These watersheds generally respond more quickly, with relatively higher peak streamflows for a given amount of rainfall excess than watersheds in other regions. Furthermore, the streamflow is often ephemeral or intermittent, because of a lack of soil moisture storage, deep groundwater, and relatively low and frequently sporadic precipitation input.

A perennial stream, that is, a stream that flows throughout the year, is likely to be sustained by groundwater. This component sustains streamflow between periods of precipitation. Because of the long pathways involved and the slow movement of subsurface flow, groundwater flow does not respond quickly to rainfall.

One characteristic of stream channels in dryland regions is high transmission losses within the channels. When stream channels are dry most of the year, much of the water moving through the systems in a runoff event can infiltrate into the channel. This water is lost from surface streams and ends up as bank storage or percolates into lower soil storage or groundwater systems. As water moves farther downstream, the volumes of water in the channel can diminish until there no longer is flow in the channel at some point downstream.

Application of the Water Budget Concept

Application of the water budget concept to study the hydrologic behavior of a watershed is relatively simple. As mentioned above, if all but one component of the hydrologic cycle is measured or estimated accurately, it then is possible to solve directly for the unknown component.

An annual water budget for a watershed is often used in an analysis because of the simplifying assumption that changes in storage in the watershed system in a year generally are small. Water budget computations can be made, beginning and ending with "wet" months or "dry" months on the watershed. In either case, the difference in storage between the beginning and end of the year's period is relatively small and, as a result, can be ignored in the calculations. By measuring the precipitation input (P) and streamflow (Q) for a year, annual evapotranspiration (ET) can be estimated from:

ET = P - Q

Provided that an "acceptable" measurement of precipitation is obtained, a second assumption made in studying a water budget is that the total outflow of water from the watershed has been measured. It is often assumed that there is no loss of water by deep seepage to underground geological strata, and that all of the groundwater flow from the watershed is measured at a gauging station. However, transmission losses can be relatively high and, when geologic strata such as limestone underlie a watershed, the surface boundaries might not coincide with the boundaries governing the flow of groundwater. There are two unknowns in the water budget in such instances, ET and groundwater seepage (L), which result in:

ET + L = P - Q

When it is not appropriate to assume that the change in storage is small, the change must be estimated. This task is difficult, although changes in storage can often be estimated by periodical measurements of the soil water content on relatively small watersheds. Such measurements can be made gravimetrically, with neutron attenuation probes, or through the use of other methods.

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