Geography 110 - Physical Geography

Notes on hydrology and water resources

Our primary concern this week is to cover basic aspects of the global water balance; the major components of the hydrologic cycle; the concept of the local soil-water budget, which in turn exerts control on the amount of surface runoff; and some important characteristics of groundwater, aquifers, and the interaction between subsurface flow and surface water. Throughout this discussion the connection between the hydrologic cycle and the global distribution of climates will be emphasized.

The global water balance involves the cycling of water between the three major compartments (ocean, atmosphere, and continents) and comparison of the annual average volumes of precipitation, evaporation, and runoff that are exchanged among these compartments.

The major elements of the hydrologic cycle are most easily illustrated using a cross-section or block diagram of a typical hillslope and stream valley, and these elements include:

The textbook follows up on its basic introduction of the hydrologic cycle with a detailed treatment of the accounting procedures for tracking the amount of moisture stored in the soil. Water reaching the land surface may infiltrate into the soil, and some of that water may percolate down into the saturated zone to become groundwater. But most groundwater will eventually re-emerge as runoff in a nearby stream channel (we will discuss this in class), so water-budget accounting generally ignores the role of groundwater and treats it as a neutral component of the budget.  The key idea is that the soil is treated as a bank or reservoir with finite capacity for storing water. During periods when precipitation (P) > evapotranspiration (ET), moisture will be stored in the soil until its moisture-holding capacity is filled,with any excess or surplus becoming runoff. Conversely, when P < ET (i.e. as in a typical summer in Maryland), moisture will be extracted from the soil by plant roots to make up the deficit. If the supply of moisture in the soil were infinite, then evapotranspiration rates would be governed only by atmospheric conditions and would occur at the "potential" rate (referred to in the book as POTET). But as the soil dries out, there is a limit on how much moisture can be extracted by plants, and actual evapotranspiration  (ACTET) < potential evapotranspiration (POTET). In the extreme, plants cannot get enough water to meet their needs and they wilt unless they are irrigated. Completing the cycle, when P > ET after a dry period, much of the surplus water is used to recharge or replenish the soil moisture in storage.

We will not follow through on the actual calculation of the water budget, but you should be familiar with the basic concepts, the elements of the budget, and the standard measurement tools (e.g. rain gauges, evaporation pans and lysimeters) as described in the text. Understanding of the seasonal cycles of soil-moisture storage is crucial for making the connection between global climate patterns and global patterns of runoff.

You should be familiar with these factors which influence soil-moisture storage:

The comparison between spatial patterns of precipitation and potential evapotranspiration over North America (figs. 9.6, 9.8) will give you a picture of the spatial distribution of moisture availability and runoff.  The global pattern of runoff is shown in fig. 9.14.

In discussing groundwater we need to start with the basic understanding that soil and rock are porous media, and that the ability to transmit water is in large measure dependent on the nature of the pore space: the total amount of pore space as well as the size distribution and spatial arrangement of the pores. Different kinds of geologic materials may be radically different with respect to their ability for storing or transmitting water. A rock layer that is capable of storing and transmitting water in usable amounts can be described as an aquifer, whereas a rock layer that acts as a barrier to flow is characterized as an aquiclude. In nature the permeability (also known as "hydraulic conductivity") of rock layers is extremely variable, and the suggestion in the textbook that all rock layers can easily be placed in one category or the other is a bit oversimplified. To the extent that some rock layers have low enough permeability that they behave as aquicludes, however, they also may act as confining layers, which in turn leads us to characterize underlying aquifers as confined aquifers.

In discussing groundwater, you should be familiar with the following features:

We will skip the discussion of water supply and water use at the end of the chapter.