Water Budget Calculator & Conservation Planner

The water balance equation ΔS = P − R − ET (or P = R + ET + ΔS) applies conservation of mass to a watershed. P (precipitation) is total water input; R (runoff) is surface and subsurface flow leaving the basin; ET (evapotranspiration) is water returned to the atmosphere by evaporation from land and water plus transpiration from plants; ΔS (change in storage) represents net change in soil moisture, groundwater, and surface water storage. If ΔS > 0, the basin is gaining water; if ΔS < 0, it is losing stored water.

Evaporation is the physical process of liquid water converting to water vapour from bare soil, open water, and wet surfaces. Evapotranspiration (ET) is the combined total of evaporation plus transpiration - the water released by plant leaves through stomata as part of photosynthesis and metabolic processes. In vegetated watersheds, transpiration can account for 70% or more of total ET. Reference ET (ET₀) is calculated for a hypothetical 0.12 m tall grass crop under standard conditions using the FAO-56 Penman-Monteith equation.

The rational method (Q = C×i×A/360) estimates peak stormwater runoff rate from small urban catchments (generally &lt;80 hectares). Q is peak discharge in m³/s, C is the dimensionless runoff coefficient (reflecting land use and soil permeability), i is rainfall intensity in mm/hr for a storm of duration equal to the time of concentration, and A is catchment area in hectares. It is widely used for storm drain design, culvert sizing, and stormwater management in urban planning.

The Green-Ampt model is a physically-based infiltration model that treats the soil as having a sharp wetting front moving downward. It computes infiltration rate f = Ks × [1 + (ψ × Δθ)/F(t)], where Ks is saturated hydraulic conductivity, ψ is the capillary suction head at the wetting front, Δθ is the initial moisture deficit (porosity minus initial moisture content), and F(t) is cumulative infiltration depth. As F increases, the infiltration rate decreases toward Ks. It is more accurate than the simpler Philip or Horton models for soils where capillary suction is important.

A watershed (also called a catchment, drainage basin, or river basin) is a land area defined by topography where all precipitation drains to a common outlet point (river, lake, or ocean). A well is an engineered structure drilled or dug into an aquifer to extract groundwater. Watersheds are delineated using topographic maps or digital elevation models (DEMs) and are the fundamental spatial unit for water resources management and hydrological analysis.

Use: V = P × A × C_r − (n × V_ff). V is harvestable annual volume in litres, P is annual rainfall in metres, A is roof catchment area in m², C_r is the runoff coefficient for the roof surface (0.7–0.95 for tiles and metal, 0.5–0.7 for rough surfaces), n is the number of rain events per year, and V_ff is the first-flush discard volume per event in litres (typically 0.2–0.4 L per m² of roof per event). The first-flush removes the most polluted initial runoff which carries bird droppings, dust, and roof debris.

Water scarcity occurs when freshwater demand exceeds available supply, or when poor quality makes water unusable. It is measured per capita: absolute scarcity &lt;500 m³/person/year, scarcity 500–1000, stress 1000–1700. Causes: population growth, agriculture (70% of global freshwater use), climate change altering precipitation patterns, pollution reducing usable supplies, poor water governance, and infrastructure gaps. Over 2 billion people live in countries experiencing water stress, according to UNEP.

ET is typically the largest water loss component in most terrestrial water budgets, often representing 60–80% of annual precipitation in humid climates and &gt;100% of precipitation in arid regions (drawn from storage). Understanding ET is critical for irrigation scheduling, drought monitoring, and hydrological modelling. Globally, ET returns about 40% of precipitation back to the atmosphere, making it a key driver of regional climate patterns and feedback loops.

Evaporation (liquid water converting to vapour driven by solar energy and vapour pressure gradients) and transpiration (water drawn upward through plant roots, stems, and leaves and released as vapour through stomata). Both processes move water from Earth's surface upward into the atmosphere, directly counteracting gravity. These are often collectively called evapotranspiration (ET) in hydrological analysis.

Urbanisation (replacing vegetation and permeable soil with impervious surfaces) dramatically increases runoff (C rises from 0.1 to 0.9), reduces infiltration, decreases ET (less vegetation), and reduces groundwater recharge. Deforestation increases runoff and reduces ET. Agricultural irrigation increases ET above natural levels. Reforestation increases ET, infiltration, and groundwater recharge while reducing peak runoff. These land-use feedbacks are central to integrated watershed management planning.

Floodplain management is a set of strategies to reduce flood risk, protect ecosystems, and guide land use in areas adjacent to rivers that are subject to periodic inundation. Measures include: zoning regulations (restricting development in high-risk zones), detention basins (ponding areas that attenuate peak flows), channel modifications (levees, bypass channels), natural flood management (restoring floodplain connectivity, wetland restoration), and early warning systems. FEMA's National Flood Insurance Program (NFIP) in the US maps flood zones and requires floodplain management ordinances for participating communities.

Infiltration rate is the speed at which water enters the soil surface, measured in mm/hr or cm/hr. It is highest at the start of a storm (when soil is dry) and decreases toward the saturated hydraulic conductivity Ks as soil wets up. Measured in the field using a double-ring infiltrometer (two concentric steel rings driven into soil, water maintained in both rings, inner ring rate measured) or by a tension infiltrometer. Typical values: sand 25–250 mm/hr, loam 10–25 mm/hr, clay &lt;5 mm/hr.

Effective household water conservation measures include: installing low-flow fixtures (shower heads &lt;8 L/min, dual-flush toilets &lt;6 L/flush); fixing leaks (a dripping tap wastes 11,000 L/year); rainwater harvesting for garden use; grey water recycling (reuse shower/sink water for toilet flushing); drought-tolerant landscaping (xeriscaping reduces outdoor water use by 50–75%); using dishwashers/washing machines only when full; and timing irrigation in the early morning or evening to reduce evaporation losses.

IWRM is a process that promotes the coordinated development and management of water, land, and related resources to maximise economic and social welfare without compromising the sustainability of vital ecosystems. It recognises: water is a finite shared resource; water has economic, social, and environmental value; all sectors (agriculture, energy, urban, ecosystems) must be coordinated; governance should involve all stakeholders. IWRM was formalised at the Dublin Principles (1992) and integrated into the UN Sustainable Development Goals (SDG 6: Clean Water and Sanitation).

Groundwater is freshwater stored in aquifers (porous rock or sediment formations) below the water table. It is recharged by infiltration from precipitation, rivers, and lakes (the G recharge term in the full water balance). Groundwater discharge occurs to rivers (baseflow), springs, and through pumped wells. Many rivers maintain flow during dry periods through groundwater baseflow. Overextraction of groundwater beyond recharge rates causes water table decline, land subsidence, and aquifer depletion - a critical global sustainability issue with 30% of global freshwater withdrawals from non-renewable aquifer storage.