Groundwater Recharge

Runoff coefficient sets what fraction of precipitation becomes surface runoff rather than infiltrating into the ground, so a higher coefficient — representing a more paved, compacted, or steeply sloped site — directly increases the runoff term the calculator subtracts from precipitation, leaving less water available to become recharge. This is exactly why urban development is widely understood to reduce natural groundwater recharge: paving over permeable ground raises the effective runoff coefficient for that land, even though total rainfall hasn't changed at all.

Runoff coefficient is primarily driven by land cover and slope: well-vegetated, flat, sandy sites commonly run around 0.1-0.2, mixed agricultural or moderately sloped land often falls in the 0.3-0.5 range, and highly paved or compacted urban surfaces can approach 0.8-0.9. Published runoff coefficient tables organized by land use and soil type (similar in spirit to the NRCS curve number tables used for stormwater runoff) are the standard reference for picking a defensible number rather than guessing, especially for anything beyond a rough back-of-envelope estimate.

Recharge as a depth (mm or m per year) is the natural unit for comparing sites of different sizes on an apples-to-apples basis, while a volume per square kilometer or per hectare is what you actually need if you're estimating total water available to an aquifer under a specific parcel or watershed. Reporting recharge as a percentage of total precipitation is useful for a quick sanity check against regional norms, since typical recharge percentages are commonly cited in hydrology literature for different climate and soil combinations. Monthly Avg Recharge is simply the annual figure divided evenly by 12 — a bookkeeping convenience, not a seasonal forecast, since real recharge is concentrated in wet months and can be near zero the rest of the year. Infiltration Efficiency is a different ratio entirely: the share of infiltrated water (precipitation minus runoff) that becomes deep recharge rather than being held in the root zone and lost back to the atmosphere via evapotranspiration.

No — the assumption that soil moisture storage doesn't change from year to year is a long-term average simplification that smooths out real year-to-year variability, so a single unusually wet or dry year can see actual recharge deviate substantially from what this formula predicts for that specific year. The method works best applied to long-term average annual precipitation, evapotranspiration, and runoff figures rather than to a single year's rainfall total, which is why hydrologists typically use it for planning-level, multi-year average estimates rather than single-season forecasting.

Because evapotranspiration plus surface runoff consumed all of the site's precipitation for the year, and this water-balance method has no basis for reporting negative recharge, so the result floors at zero rather than showing a deficit. This is a real, expected outcome for arid and semi-arid sites where annual evapotranspiration is high relative to precipitation (a desert climate, for example) — it means this simplified annual method estimates essentially no net percolation to the water table from precipitation alone that year, not that the calculator has malfunctioned.