Water Budgets and Resource Analysis Flashcards
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Water budget (9) A water accounting system for an area of Earth’s surface using inputs of precipitation and outputs of evapotranspiration (evaporation from ground surfaces and transpiration from plants) and surface runoff. Precipitation “income” balances evaporation, transpiration, and runoff “expenditures”; soil moisture storage acts as “savings” in the budget.
An effective method for assessing portions of the water cycle as they apply to water resources is to establish a water budget for any area of Earth’s surface—a continent, country, region, field, or front yard. A water budget is derived from measuring the input of precipitation and its distribution and the outputs of evapotranspiration, including evaporation from ground surfaces and transpiration from plants, and surface runoff. Also included in this budget is moisture that is stored in the soil-moisture zone. Such a budget can cover any time frame, from minutes to years.
What is a surplus in the water budget?
Surplus (9)
S; the amount of moisture that exceeds potential evapotranspiration; moisture oversupply when soil-moisture storage is at field capacity; extra or surplus water.
Sometimes all expenditure demands are met, and any extra water results in a surplus.
What is a deficit in the water budget?
Deficit (9)
D in a water balance, the amount of unmet (unsatisfied) potential evapotranspiration (PE); a natural water shortage. (See Potential evapotranspiration.)
At other times, precipitation and soil-moisture savings are inadequate to meet demands, and a deficit, or water shortage, results.
What is precipitation?
The moisture supply to Earth’s surface is precipitation (P) in all its forms, such as rain, snow, or hail.
Precipitation (9)
Rain, snow, sleet, and hail—the moisture supply; called P in the water balance.
What is a Rain gauge?
Rain gauge (9) A weather instrument; a standardized device that captures and measures rainfall.
One way to measure precipitation is with a rain gauge, essentially a large measuring cup that collects rainfall and snowfall so the water can be measured by depth, weight, or volume
What is Potential Evapotranspiration?
Potential evapotranspiration (9) PE; the amount of moisture that would evaporate and transpire if adequate moisture were available; it is the amount lost under optimum moisture conditions, the moisture demand. (Compare Actual evapotranspiration.)
Evapotranspiration is an actual expenditure of water to the atmosphere. In contrast, potential evapotranspiration (PE) is the amount of water that would evaporate and transpire under optimum moisture conditions when adequate precipitation and soil moisture are present.
What is Actual Evapotranspiration?
Actual evapotranspiration (9) AE; the actual amount of evaporation and transpiration that occurs; derived in the water balance by subtracting the deficit (D) from potential evapotranspiration (PE).
If we subtract the deficit from the potential evapotranspiration, we derive what actually happened—actual evapotranspiration (AE).
What is an evaporation pan/evaporimeter?
Precise measurement of evapotranspiration is difficult. One method employs an evaporation pan, or evaporimeter. As evaporation occurs, water in measured amounts is automatically replaced in the pan, equalling the amount that evaporated
What is a Lysimeter?
A more elaborate measurement device is a lysimeter, which isolates a representative volume of soil, subsoil, and plant cover to allow measurement of the moisture moving through the sampled area.
What is Soil-moisture storage?
Soil-moisture storage (9)
STRGE; the retention of moisture within soil; it is a savings account that can accept deposits (soil-moisture recharge) or allow withdrawals (soil-moisture utilization) as conditions change.
As part of the water budget, the volume of water in the subsurface soil-moisture zone that is accessible to plant roots is soil-moisture storage (ST). This is the savings account of water that receives deposits (or recharge) and provides for withdrawals (or utilization).
What are the three categories of water that the soil-moisture environment includes?
The soil-moisture environment includes three categories of water—hygroscopic, capillary, and gravitational. Only hygroscopic and capillary water remain in the soil-moisture zone; gravitational water fills the soil pore spaces and then drains downward under the force of gravity. Of the two types of water that remain, only capillary water is accessible to plants
What is Hygroscopic water?
Hygroscopic water (9) That portion of soil moisture that is so tightly bound to each soil particle that it is unavailable to plant roots; the water, along with some bound capillary water, is left in the soil after the wilting point is reached. (See Wilting point.)
When only a small amount of soil moisture is present, it may be unavailable to plants. Hygroscopic water is inaccessible to plants because it is a molecule-thin layer that is tightly bound to each soil particle by the hydrogen bonding of water molecules. Hygroscopic water exists in all climates, even in deserts, but it is unavailable to meet PE demands.
What is the Wilting point?
Wilting point (9) That point in the soil-moisture balance when only hygroscopic water and some bound capillary water remain. Plants wilt and eventually die after prolonged stress from a lack of available water.
Soil moisture is at the wilting point for plants when all that remains is this inaccessible water; plants wilt and eventually die after a prolonged period at this degree of moisture stress.
What is Capillary water?
Capillary water (9) Soil moisture, most of which is accessible to plant roots; held in the soil by the water’s surface tension and cohesive forces between water and soil. (See also Field capacity, Hygroscopic water, Wilting point.)
Capillary water is generally accessible to plant roots because it is held in the soil, against the pull of gravity, by hydrogen bonds between water molecules (that is, by surface tension), and by hydrogen bonding between water molecules and the soil. Most capillary water is available water in soil-moisture storage.
What is Field Capacity?
Field capacity (9) Water held in the soil by hydrogen bonding against the pull of gravity, remaining after water drains from the larger pore spaces; the available water for plants. (See Capillary water.)
After some water drains from the larger pore spaces, the amount of available water remaining for plants is termed field capacity, or storage capacity. This water can meet PE demands through the action of plant roots and surface evaporation.
