2. Water balance concept

Understanding the Water Balance Concept

1. Introduction

The water balance is a fundamental concept in hydrology that describes the inflow, outflow, and storage changes in a given hydrological system over a specified period. It is based on the law of conservation of matter, ensuring that all water entering a system is accounted for either as storage or as an output. Understanding water balance is essential for water resource management, flood forecasting, and irrigation planning.

---

2. Catchment Area and Water Budget Equation

2.1 Catchment Area

A catchment area is the land region draining into a stream or watercourse at a specific location. It is also referred to as:

• Drainage area or drainage basin.

• Watershed (USA terminology).

• Divide (USA) or watershed (UK) separates one catchment from another.

A catchment's hydrology is influenced by various components such as precipitation, infiltration, and surface runoff.

2.2 Water Budget Equation

The water balance equation, also known as the hydrologic equation, follows the principle of mass conservation:

Where:

• I = Inflow

• O = Outflow

• ΔS = Change in storage over a given time period

The volumes are often expressed as average depths over the catchment area. For instance, if an annual streamflow from a 10 km² catchment is 10⁷ m³, this corresponds to a depth of:

---

3. Components of Water Balance

The expanded water budget equation for a catchment area is:

Where:

• P = Precipitation

• R = Surface runoff

• G = Net groundwater outflow

• E = Evaporation

• T = Transpiration

• ΔS = Change in storage

3.1 Inflow Components (I)

1. Precipitation (rainfall, snow, hail, dew)

2. Surface inflow (water entering from upstream sources)

3. Subsurface inflow (groundwater flow into the system)

4. Imported water or sewage through pipelines or channels

3.2 Outflow Components (O)

1. Surface runoff (water flowing over land into streams and rivers)

2. Evaporation (water loss from soil, lakes, and reservoirs)

3. Subsurface runoff (water draining out through the ground)

4. Evapotranspiration (combined water loss from evaporation and plant transpiration)

5. Transpiration (water loss through plant stomata)

6. Exported water or sewage (man-made diversions)

3.3 Storage Change (ΔS)

The change in storage consists of:

• ΔSₛ = Change in surface water storage

• ΔSₘ = Change in soil moisture storage

• ΔSₑ = Change in groundwater storage

The relationship between rainfall and runoff can be expressed as:

Where:

• R = Runoff

• P = Precipitation

• L = Losses (evaporation, transpiration, infiltration, etc.)

---

4. Applications of Water Balance Concept

4.1 Water Resource Management

• Helps design sustainable water supply and distribution systems.

• Assists in managing reservoirs and optimizing groundwater recharge.

4.2 Hydrological Modeling

• Used in flood forecasting, drought assessments, and climate impact studies.

• Applied in models like SWAT (Soil and Water Assessment Tool) and HEC-HMS.

4.3 Irrigation Planning

• Guides efficient irrigation water allocation based on soil moisture balance.

• Prevents over-extraction of groundwater.

4.4 Urban Stormwater Management

• Aids in designing drainage systems to prevent waterlogging and urban flooding.

• Supports green infrastructure development such as rainwater harvesting.

4.5 Climate Change Studies

• Evaluates long-term shifts in precipitation, runoff, and water storage.

• Helps in adapting water management strategies for extreme weather conditions.

Calculations

Let's assume the following values for a small catchment over a month:

• Precipitation (P) = 120 mm
• Runoff (R) = 30 mm
• Evaporation (E) = 20 mm
• Transpiration (T) = 15 mm
• Groundwater outflow (G) = 10 mm
• Change in storage (dS) = ? (to be calculated)

The change in storage for the catchment is +45 mm, indicating an increase in water stored within the catchment over the month.