As water resources have become more scarce, greater demands have been placed upon those resources. It is no longer a function of just collecting, storing, and transporting water from places were it is accumulated. We are now balancing these forces with those of water quality, and environmental impacts such as droughts and floods. The science of hydrology is gradually changing as we struggle to control nature's fluctuations, and our ever increasing demands upon the waters of the earth.

First a few definitions:
Hydrology is that portion of science which deals specifically with the properties, distribution, and circulation of water in the atmosphere and on and in the earth

Hydrography is that portion of science which describes, measures, and maps surface waters (in particular those used for navigational purposes).

A hydrograph is a graph or computer-generated trend screen which depicts the rate of flow or water level of a body of water in relationship to a given period of time (usually of sufficient time to record seasonal changes.)

Water Bank (Budget): A Mass Balance for Water
We find that the environmental sources around us have great cyclical fluctuations. In the years of abundance this is not a problem. It is those years when water is in short supply that we seek to modify our watersheds in such a manner to store water during wet periods and use this stored water to dampen the effects of drought situations.

In order to accomplished this we must know the quantity of water that are watershed is receiving and how much water we're losing. Simply put, we're dealing with a "water bank" or "water budget." This is a mass balance of the water quantities.

A water bank may be defined as:
(All inflows) - (all outflows) = variation in storage

Water may be stored for single or multiple uses. Multiple uses include recreational, agricultural, industrial, residential, hydroelectric power, and flood control.

Inflows include all water sources from atmospheric precipitation and groundwater supplies. It also includes any artificial inflows such as piping in water from another watershed.

Outflows include evaporation, plant material transpiration, non-recoverable soil infiltration losses and surface runoff.

It is obvious that human activities impact almost all of these functions to some extent.

Some water uses are "consumptive" such as food processing, loses to the growth of plant material, losses due to being incorporated into a product, etc. Water which has become polluted during its use, and cannot be economically treated, is an obvious consumptive water use.

Other water uses may be just that: water uses that, after use, return a large percentage of the source water back to the watershed.

Hydrology Component Parts
Hydrology may be easily broken down into several component parts:
a) hydrology of flowing water
b) hydrology of impounded water
c) hydrology of groundwater

Each of these component parts is practically a science unto itself. Each involves the accumulation of information and data; and the creation of models, mathematical calculations, and other tools to define, describe, and predict hydrological events. (The picture of the dam to the left creates Lake Berryessa, in Northern California. Once the lake is full, "the excess water" goes over into the "Glory Hole" and falls down a 200 foot pipe, flows thru the rock side in a tunnel, and shoots out the side of the rock face as shown. Hydroelectric power is also a feature of this project.)

Hydrology Activities
Hydrology activities include some of the following:
measuring precipitation; and determining the probability of precipitation magnitudes and number of events in a specified period of time; stream flow calculations involving quantity of flow, and percentage of run off; reservoir and lake evaporation and percolation losses, capacities, and yields; infiltration rates to groundwater aquifers, withdrawl rates, and safe yield.

Hydrological studies require a complex, time-consuming process of accumulating environmental watershed knowledge such as: topography, soil types, geology, biological, precipitation rates, etc. The image shows how siltation can influence a water storage reservoir by "filling in" the reservoir capacity with the silt brought in from the watershed, and also impacting raw water quality with higher turbidities, higher organics, etc.

The more extensive studies include present agricultural, residential, and industrial development along with future growth in each of these areas.

Surface water runoff that is not controlled nor impacted by human interference is called " natural runoff."

Surface water runoff that is controlled or impacted by human activities is called " regulated runoff."

Hydrologic Residence Time:
Just as a treatment process has a hydraulic residence time, the residence time for a reservoir, lake, or even a watershed, may be calculated in the same manner. The hydraulic residence time may be calculated by dividing the volume of the water impoundment by the quantity of water leaving the impoundment.

Hrt, days = Volume/Outflow

As there are many approaches and varying definitions of this simple calculation, it is very important to describe completely how this calculation has been derived.


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