Managing Semi-Arid Watersheds: Watershed Basics - The Hydrologic Cycle

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

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 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—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.