Water Balance: Meaning, Components and Types | Hydrology | Geology

In this article we will discuss about:- 1. Meaning of Water Balance 2. Components of Water Balance 3. Types.

Meaning of Water Balance:

The water balance is an accounting of the inputs and outputs of water. The water balance of a place, whether it is an agricultural field, watershed, or continent, can be determined by calculating the input, output, and storage changes of water at the Earth’s surface.

The major input of water is from precipitation and output is evapotranspiration. The geographer C. W. Thornthwaite (1899-1963) pioneered the water balance approach to water resource analysis. He and his team used the water-balance methodology to assess water needs for irrigation and other water-related issues.

A water balance diagram will help your organisation to understand water use and may help you to reduce water costs.

The diagram will include known water use, help to identify leaks, overuse and areas where efficiency improvements could be made. This could include the installation of meters (sub-metering) at significant points in the system.

Components of Water Balance:

To understand water-balance concept, we need to start with its various components:

i. Precipitation (P):

Precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena) is any product of the condensation of atmospheric water vapour that is pulled down by gravity and deposited on the Earth’s surface. The main forms of precipitation include rain, snow, ice pellets, and graupel. It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water condenses, falling out of solution (i.e., precipitates).

Two processes, possibly acting together, can lead to air becoming saturated – cooling the air or adding water vapour to the air. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface; it is one of the ways air can become saturated. Precipitation forms via collision with other rain drops or ice crystals within a cloud.

Moisture overriding associated with weather fronts is an overall major method of precipitation production. If enough moisture and upward motion is present, precipitation falls from convective clouds such as cumulonimbus and can organize into narrow rain-bands. Where relatively warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extra-tropical cyclones. Lake-effect snowfall can be locally heavy.

Thunder-snow is possible within a cyclone’s comma head and within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. The movement of the monsoon trough, or inter-tropical convergence zone, brings rainy seasons to savannah climes.

Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops. Precipitation that reaches the surface of the earth can occur in many different forms, including rain, freezing rain, drizzle, ice needles, snow, ice pellets or sleet, graupel and hail. Hail is formed within cumulonimbus clouds when strong updrafts of air cause the stones to cycle back and forth through the cloud, causing the hailstone to form in layers until it becomes heavy enough to fall from the cloud.

Unlike raindrops, snowflakes grow in a variety of different shapes and patterns, determined by the temperature and humidity characteristics of the air the snowflake moves through on its way to the ground. While snow and ice pellets require temperatures close to the ground to be near or below freezing, hail can occur during much warmer temperature regimes due to the process of its formation. Precipitation may occur on other celestial bodies, e.g., when it gets cold, Mars has precipitation which most likely takes the form of ice needles, rather than rain or snow.

ii. Actual Evapotranspiration (AE):

Evaporation is the phase change from a liquid to a gas releasing water from a wet surface into the air above. Similarly, transpiration is represents a phase change when water is released into the air by plants. Evapotranspiration is the combined transfer of water into the air by evaporation and transpiration. Actual evapotranspiration is the amount of water delivered to the air from these two processes. Actual evapotranspiration is an output of water that is dependent on moisture availability, temperature and humidity.

Think of actual evapotranspiration as “water use”, that is, water that is actually evaporating and transpiring given the environmental conditions of a place. Actual evapotranspiration increases as temperature increases, as long as there is water to evaporate and for plants to transpire. The amount of evapotranspiration also depends on how much water is available, which depends on the field capacity of soils. In other words, if there is no water, no evaporation or transpiration can occur.

iii. Potential Evapotranspiration (PE):

The environmental conditions at a place create a demand for water. Especially in the case for plants, as energy input increases, so does the demand for water to maintain life processes. If this demand is not met, serious consequences can occur. If the demand for water far exceeds that which is actual present, dry soil moisture conditions prevail. Natural ecosystems have adapted to the demands placed on water.

Potential evapotranspiration is the amount of water that would be evaporated under an optimal set of conditions, among which is an unlimited supply of water. Think of potential evapotranspiration of “water need”. In other words, it would be the water needed for evaporation and transpiration given the local environmental conditions. One of the most important factors that determine water demand is solar radiation.

As energy input increases the demand for water, especially from plants increases. Regardless if there is, or isn’t, any water in the soil, a plant still demands water. If it doesn’t have access to water, the plant will likely wither and die.

iv. Soil Moisture Storage (ST):

Soil moisture storage refers to the amount of water held in the soil at any particular time. The amount of water in the soil depends on soil properties like soil texture and organic matter content. The maximum amount of water the soil can hold is called the field capacity. Fine grain soils have larger field capacities than coarse grain (sandy) soils. Thus, more water is available for actual evapotranspiration from fine soils than coarse soils. The upper limit of soil moisture storage is the field capacity, the lower limit is 0 when the soil has dried out.

v. Change in Soil Moisture Storage (ÄST):

The change in soil moisture storage is the amount of water that is being added to or removed from what is stored. The change in soil moisture storage falls between 0 and the field capacity.

a. Deficit (D):

A soil moisture deficit occurs when the demand for water exceeds that which is actually available. In other words, deficits occur when potential evapotranspiration exceeds actual evapotranspiration (PE > AE). Recalling that PE is water demand and AE is actual water use (which depends on how much water is really available), if we demand more than we have available we will experience a deficit. But, deficits only occur when the soil is completely dried out. That is, soil moisture storage (ST) must be 0. By knowing the amount of deficit, one can determine how much water is needed from irrigation sources.

b. Surplus (S):

Surplus water occurs when P exceeds PE and the soil is at its field capacity (saturated). That is, we have more water than we actually need to use given the environmental conditions at a place. The surplus water cannot be added to the soil because the soil is at its field capacity so it runs off the surface. Surplus runoff often ends up in nearby streams causing stream discharge to increase. A knowledge of surplus runoff can help forecast potential flooding of nearby streams.

c. Surface Runoff:

Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, snowmelt, or other sources flows over the land. This is a major component of the hydrologic cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains man-made contaminants, the runoff is called nonpoint source pollution.

A land area which produces runoff that drains to a common point is called a watershed. When runoff flows along the ground, it can pick up soil contaminants such as petroleum, pesticides (in particular herbicides and insecticides), or fertilizers that become discharge or non-point source pollution.

Surface runoff can be generated either by rain fall or by the melting of snow, ice, or glaciers. Snow and glacier melt occur only in areas cold enough for these to form permanently. Typically snowmelt will peak in the spring and glacier melt in the summer, leading to pronounced flow maxima in rivers affected by them. The determining factor of the rate of melting of snow or glaciers is both air temperature and the duration of sunlight. In high mountain regions, streams frequently rise on sunny days and fall on cloudy ones for this reason.

Computing a Soil – Moisture Budget:

The best way to understand how the water balance works is to actually calculate a soil water budget. We’ll use Rockford, Illinois which is located in the humid continental climate of northern Illinois. Rockford lies on the northern edge of the prairie and mixes with deciduous forest. This vegetation has been nearly completely replaced with agriculture. A knowledge of soil moisture status is important to the agricultural economy of this region that produces mostly corn and soy beans.

To work through the budget, we’ll take each month (column) one at a time. It’s important to work column by column as we’re assessing the moisture status in a given month and one month’s value may be determined by what happened in the previous month.

Soil Moisture Surplus:

During December, Rockford is deep in the grip of winter. Potential evapotranspiration has dropped to zero as plants have gone into a dormant period thus reducing their need for water and cold temperatures inhibit evaporation. Notice that P-PE is equal to 45 but not all is placed into storage. Why? At the end of November the soil is within 12 mm of being at its field capacity. Therefore, only 12 millimeters of the 45 available is put in the soil and the remainder runs off as surplus (S = 33).

Given that the soil has reached its field capacity in December, any excess water that falls on the surface in January will likely generate surplus runoff. According to the water budget table this is indeed true. Note that P-PE is 50 mm and ÄST is 0 mm. What this indicates is that we cannot change the amount in storage as the soil is at its capacity to hold water. As a result the amount is storage (ST) remains at 90 mm. Being a wet month (P > PE) actual evapotranspiration is equal to potential evapotranspiration. Note that all excess water (P-PE) shows up as surplus (S = 50 mm).

Similar conditions occur for the months of February, March, April, and May. These are all wet months and the soil remains at its field capacity so all excess water becomes surplus. Note too that the values of PE are increasing through these months. This indicates that plants are springing to life and transpiring water. Evaporation is also increasing as insolation and air temperatures are increasing. Notice how the difference between precipitation and potential evapotranspiration decreases through these months.

As the demand on water increases, precipitation is having a harder time satisfying it. As a result, there is a smaller amount of surplus water for the month. Surplus runoff can increase stream discharge to the point where flooding occurs. The flood duration period lasts from December to May (6 months), with the most intense flooding is likely to occur in March when surplus is the highest (61 mm).

Soil Moisture Utilization:

By the time June rolls around, temperatures have increased to the point where evaporation is proceeding quite rapidly and plants are requiring more water to keep them healthy. As potential evapotranspiration is approaching its maximum value during these warmer months, precipitation is falling off. During June P-PE is -17 mm.

What this means is precipitation no longer is able to meet the demands of potential evapotranspiration. In order to meet their needs, plants must extract water that is stored in the soil from the previous months. This is shown in the table by a value of 17 in the cell for ÄST (change in soil storage). Once the 17 m is taken out of storage (ST) it reduces its value to 73.

The month of June is considered a dry month (P < PE) so AE is equal to precipitation plus the absolute value of ÄST (P + |ÄST|). When we complete this calculation (106 mm + 17 mm = 123 mm) we see that AE is equal to PE. What this means is precipitation and what was extracted from storage was able to meet the needs demanded by potential evapotranspiration.

Note that there is no surplus in June as the soil moisture storage has dropped below its field capacity. There is still no deficit as water remains in storage. The calculations for July is similar to June, just different values. Note that by the time July ends, water held in storage is down to a mere 16 mm.

Soil Moisture Deficit:

August, like June and July, is a dry month. Potential evapotranspiration still exceeds precipitation and the difference is a – 42 mm. Up until this month there has been enough water from precipitation and what is in storage to meet the demands of potential evapotranspiration. So, of the 42 mm of water we would need (P – PE) to extract from the soil.

In so doing, the amount in storage (ST) falls to zero and the soil is dried out. What happens to the remaining 26 mm of the original P-PE of 42? The unmet need for water shows up as soil moisture deficit. In other words, we have not been able to meet our need for water from both precipitation and what we can extract from storage. AE is therefore equal to 100 mm (84 mm of precipitation plus 16 mm of ÄST).

So what is a farmer to do if their crops cannot obtain needed water from precipitation or soil moisture storage….they irrigate. Irrigation water usually is pumped from groundwater supplies held in aquifers deep below the surface or from nearby streams (if stream flow is sufficient to provide needed water). The amount of irrigation water required is the amount of the soil moisture deficit.

Soil Moisture Seasons:

Four soil moisture seasons can be defined by the soil moisture conditions.

Recharge:

The recharge season is a time when water is added to soil moisture storage (+∆ST). The recharge period occurs when precipitation exceeds potential evapotranspiration but the soil has yet to reach its field capacity.

Surplus:

The surplus season occurs when precipitation exceeds potential evapotranspiration and the soil has reached its field capacity. Any additional water applied to the soil runs off. If this water runs off into nearby streams and rivers it could cause flooding. Thus, the intensity (amount) and duration (length of season) of surplus can be used to predict the severity of potential flooding.

Utilization:

The utilization season is a time when water is withdrawn from soil moisture storage (–ÄST). The utilization period occurs when potential evapotranspiration exceeds precipitation but soil storage has yet to reach 0 (dry soil).

Deficit:

The deficit season occurs when occurs when potential evapotranspiration exceeds precipitation and soil storage has reached 0. This is a time when there is essentially no water for plants. Farmers then tap ground water reserves or water in nearby streams and lakes to irrigate their crops. Thus, the intensity (amount) and duration (length of season) of deficit can be used to predict the need for irrigation water.

Whether a place experiences all four seasons depends on the climate and soil properties. Wet climate and those places with soils having high field capacities are less likely to experience a deficit period. Likewise the duration and intensity of any season will be determined by the climate and soil properties. Given equal amounts of precipitation, coarse textured soils will generate runoff faster than fine textured soils and may experience more intense surplus.

Type of Surface Water Balance:

The water balance of the entire mine, a number of components, or a single entity, such as the heap leach pad, may be quantified as part of the water quality and/or quantity management activities at a mine site.

Reasons for undertaking a facility or site water balance study may include:

(a) Evaluate strategies for optimum use of limited water supplies;

(b) Establish procedures for limiting site discharge and complying with discharge requirements, particularly control of the quality of the water and/or the quantity of contaminants discharged from the site; and

(c) Limiting or controlling erosion due to flow over exposed surfaces or in channels, swales, and creeks; and

(d) Estimating the demands on water treatment plants, holding ponds, evaporation ponds, or wetlands.

Analytical Approaches:

The most common way to build a water balance model of a facility of site is to use that famous stand-by, Excel. The reason is that most water balance models generally involve no more than successive solution for each component of a facility and hence for each facility of the simple equation –

Inflow – Outflow = Change in Storage 

Use of the Mine Water Balance Model:

The following are the steps in setting up, refining, and using a water balance model of your mine:

i. Model:

Have an effective, robust, calibrated and easily updated and adjusted water quality and quantity (volumetric flow) model to understand the complex relationships of the mine for the prediction of water changes. Model all sources of contamination and the inputs as well as outputs. The stakeholders should agree that the model is accurate and appropriate.

ii. Measure:

Have an effective sampling program to keep the water quality and quantity model up to date and continuously evaluate its effectiveness and test its assumptions.

iii. Calibrate:

Have the model checked each week initially (then monthly) with water quality and quantity numbers and monitor discrepancies between reality and model; evaluate and explain discrepancies.

iv. Contingency Plans:

Have a full set of costed contingency plans with established implementation timelines. Complete the engineering for likely long-term options.

v. Manage:

Understand all possible actions that can be taken to minimise water quality and quantity issues and have them costed to +/– 35% accuracy. Know at what levels what actions need to be taken when pre-specified levels are reached so that management can confidently make decisions which meet its license limits while incurring the least expenditure.

General Relevant Information:

Here is a brief description of the various types of data you need to model the water balance of your mine.

i. Climate:

Data are required to quantify – precipitation, snow depths and melt patterns, evaporation, evapotranspiration, wind, and solar radiation. Such data may come from one or more of many sources, including site measurement records, regional databases—usually the local airport, local and national databases accessible on the web, or synthetically generated data that many computer codes produce.

ii. Surface Water:

Data may be needed for a water balance study about local stream flow, surface runoff patterns and quantities, and infiltration patterns and rates. Establishing these quantities may again involve consulting site-specific measurement records, local data bases, or running computer codes that enable one to calculated infiltration through a soil surface such as the cover of a waste pile. The most common code for infiltration estimation is HELP, copies of which are commercially available from many vendors.

iii. Groundwater:

Groundwater flow patterns and rates must be known or predicted to model the water balance of a facility or mine. At some sites, the groundwater emerges as springs which add to the quantity (as sometimes the constituent loading) of a site. At most facilities and mines, protection of groundwater quality by limiting seepage to the groundwater is a prime objective. Quantification of groundwater flow regimes is complex even at the simplest of sites, and usually involves detailed site-specific studies based on monitoring wells and a history of water quality sampling.

iv. Facility Layout:

Before starting a water balance study it is imperative that you have good information about the site and facility layout. This includes – quantification of area, topography, runoff, slopes, location and condition of streams and man-made channels, and possibly even the layout of the mine pit itself.

Preferable the data should include digital maps that may be used with CADD systems to calculate areas, slopes, etc.

Facility Material Characteristics:

Geologist and geotechnical engineers will probably have to be involved to characterise the materials of the facilities that are part of the water balance study. The prime characteristic is, or more correctly the hydraulic conductivity, of the soils and rocks that make up the strata at the site, that constitute the mass of the waste rock dump, heap leach pad, or tailings impoundment, or which serve as the cover of reclaimed and closed waste piles. Sampling and laboratory testing quantify the hydraulic conductivity of soil and rock. In situ wells testing quantifies bedrock permeability.

v. Vegetation:

Evapotranspiration via vegetation is often the primary route by which water is lost or removed from a mine water balance system. The analyst needs to know the types and distribution of vegetation. Most computer codes that enable the analyst to quantify evapotranspiration require input of vegetation coverage, density, rooting depth, and periods of growth and quiescence. Collect such information by field observation, and supplement with studies, in situ testing, regional studies, or calibration of models by collecting data and comparing measured and calculated quantities.