Water Budget Calculator

This calculator applies the fundamental hydrologic water-balance equation -- Storage Change (deltaS) = Precipitation - Evapotranspiration - Surface Runoff - Groundwater Outflow -- to tell you whether a watershed gained or lost water in storage (soil moisture, groundwater, surface water) over the budget period. Precipitation is the only term that adds water to the system in this equation; the other three all subtract from it, and all four terms carry an equal, unweighted coefficient of magnitude 1 -- a one-millimeter change in Precipitation shifts Storage Change by exactly as much as a one-millimeter change in any single loss term, since the equation is a plain sum and difference, not a weighted one. At this calculator's default values Precipitation happens to be the numerically largest of the four (900 mm versus 500/200/50 mm for the loss terms), which is typical for a real watershed since the loss terms draw from what Precipitation supplies, but that's a property of the example numbers, not a structural weighting built into the formula. A positive Storage Change means the watershed is gaining water (a Surplus), a negative value means it's losing water faster than it's being replenished (a Deficit), and the exact break-even point depends on your specific loss totals -- at this calculator's defaults, storage tips from surplus to deficit once Surface Runoff alone exceeds 350 mm, holding the other terms fixed.

The P/ET Ratio (Precipitation divided by Evapotranspiration) shown here is a simplified wetness indicator, not the standard climate-classification Aridity Index -- the real UNEP Aridity Index divides Precipitation by potential evapotranspiration (the water a fully-watered surface could lose to the atmosphere, an independent measure of atmospheric demand) and classifies climates as humid above a 0.65 threshold and arid, semi-arid, or dry sub-humid below it. This calculator instead reuses the water balance's own actual Evapotranspiration term above (already limited by whatever water was actually available, not atmospheric demand), so read a ratio above 1 as "Precipitation exceeded this period's actual water loss to ET" and below 1 as "it didn't" -- a rough within-calculator wetness signal, not a substitute for a real climate classification. This is a simplified, lumped-parameter water budget suited to a first-pass regional or watershed-scale estimate -- it doesn't account for soil storage capacity, snowpack, or within-period timing, so a real hydrologic study would typically model these terms over shorter time steps and account for how water moves between storage compartments, not just the net change.