8.4.1: The Physical Template

The Physical Template

Within the watershed, various forms of matter, including water, are in constant cyclic flow. Through these processes, an abiotic (non-living) template of air, water, and soil is formed, upon which life can exist. The physical template of watershed structure is ultimately determined by varying combinations of climatic, geomorphic, and hydrologic processes.

Figure 8.4.18.4.1 Climatic factors. (USEPA Introduction to Watershed Ecology )

Climatology, the science of climate and its causes, becomes important in understanding regional issues in watershed science (Figure 8.4.18.4.1). Though sometimes used synonymously with weather, climate is actually a distinct term with important ecological ramifications. Climate refers to an aggregate of both average and extreme conditions of temperature, humidity, and precipitation (including type and amount), winds, and cloud cover, measured over an extended period of time. Weather refers to present day environmental conditions; current temperatures and meteorological events make up weather, not climate. Long-term weather trends establish averages which become climatic regimes. Climate heavily influences watershed vegetation communities, stream flow magnitude and timing, water temperature, and many other key watershed characteristics.

Geology is defined as the science centered around the study of various earth structures, processes, compositions, characteristics, and histories. Geomorphology, however, refers specifically to the study of the landforms on the earth and the processes that change them over time (Figure 5). Fluvial geomorphology, referring to structure and dynamics of stream and river corridors, is especially important to understanding the formation and alteration of the stream or river channel as well as the flood plain and associated upland transitional zone; this is a critical discipline for effective, long-term watershed management. Figure 3. The physical template determines watershed structure Figure 5. Geomorphology helps explain river and watershed form. Figure 4. Climatic factors

One of the life-sustaining cycles we are most familiar with is the hydrologic cycle (Figure 8.4.28.4.2). This cycle is a natural, solar-driven process of evaporation, condensation, precipitation, and runoff.

Figure 8.4.28.4.2 The hydrologic cycle. (USEPA Introduction to Watershed Ecology )

Hydrology is the science of water, as it relates to the hydrologic cycle. More specifically, it is the science of water in all its forms (liquid, gas, and solid) on, in and over the land areas of the earth, including its distribution, circulation and behavior, its chemical and physical properties, together with the reaction of the environment (including all living things) on water itself. The global water budget (Table 8.4.18.4.1) adds further insight into the water resources of our planet.

Table 8.4.18.4.1: The global water budget

Saltwater	Percent of global water		Freshwater	Percent of global water
Oceans	97.542		Glaciers/Ice	1.806
Salt lakes/seas	0.004		Ground/Soil: root zone 0-2500 feet 2500-12500 feet	0.001 0.303 0.331
			Fresh Lakes Reservoirs	0.01 <0.001
			Atmosphere	0.001
			Rivers	<0.001
Total Salt	97.546%		Total Fresh	2.454%

The three elements of the physical template and other factors also interact significantly in determining the structure and composition of a watershed and its biotic communities. In this diagram (Figure 8), the strength of influence on the watershed is represented in two ways: the relative size of the arrows, and the level of control indicated at the right side of the diagram.

Figure 8.4.38.4.3 How elements of the physical template interact. (USEPA Introduction to Watershed Ecology )

As a result of different combinations of these formative processes, different types of watersheds are created (Figure 9). Here are some examples that show how different from one another watersheds of different origin and physical template conditions can be.

Figure 8.4.48.4.4 6b. Valley Type II, moderately steep, gentle sloping side slopes often in colluvial valleys. 10b. Valley Type VI, moderately steep, fault controlled valleys. 12b. Valley Type VIII, wide, gentle valley slope with well developed floodplain adjacent to river terraces. (USEPA Introduction to Watershed Ecology )

Thomas C. O'Keefe, Scott R. Elliott and Robert J. Naiman, Introduction to Watershed Ecology, USEPA Watershed Academy Web, Accessed on December 2023, https://cfpub.epa.gov/watertrain/module