Irrigation Water Calculator

Evapotranspiration is the primary mechanism by which a crop field loses water. On a hot, dry, windy day in July, a mature corn crop can transpire 0.30 to 0.35 inches of water — more than 8,200 gallons per acre. Over a typical US Corn Belt growing season from May through September, cumulative ET for corn runs 20 to 24 inches, meaning a single 160-acre farm must replace 5 to 6 acre-feet of water annually just to meet crop demand, on top of whatever rainfall provides.

The FAO Penman-Monteith equation, published in FAO Irrigation and Drainage Paper No. 56, is the global standard for computing reference ET from weather data. It accounts for the four primary atmospheric drivers of evaporation: incoming solar radiation (the energy source), vapor pressure deficit (the drying power of the air), air temperature, and wind speed. State-level weather networks operated by NRCS, Cooperative Extension, and the Western Regional Climate Center compute and publish daily ETo from automated weather stations — many in real-time.

The reference ET calculation assumes a standardized 12-cm tall grass surface. To estimate actual crop water use, ETo is multiplied by the crop coefficient Kc for the specific crop and growth stage. The FAO-56 publication tabulates Kc for over 70 crops; this calculator uses the single-value Kc approach accurate to within ±10% for most scheduling purposes.

The Kc curve over a crop's life cycle follows a characteristic shape: low at initial planting when the canopy is sparse, rising steeply through rapid vegetative growth, peaking at full canopy cover during the mid-season phase, and declining at late season as the crop matures and senesces. This shape is why irrigation demand is not constant — a cornfield in June needs far less water than the same field in July at silking.

Mid-season Kc values for common crops reflect canopy size and stomatal behavior at peak demand: corn and alfalfa at Kc = 1.20 both have high water demand, driven by large leaf areas and frequent cutting (alfalfa) or dense closed canopies (corn). Cotton peaks at Kc = 1.15 during boll fill. Tomatoes have a mid-season Kc of 1.05. Turf grass is unusual in having essentially constant Kc ≈ 0.85 across the season because it is maintained at a constant leaf area through mowing.

Late-season Kc values below 1.0 mean the field can often be deficit-irrigated during grain fill without significant yield loss — a practice called regulated deficit irrigation that can save 15 to 20% of total season water use.

Application efficiency is the single most important factor in irrigation system economics. A center pivot at 85% efficiency delivering 2.0 inches gross applies 1.70 inches net to the root zone. The same gross application from a furrow system at 65% efficiency delivers only 1.30 inches net — meaning the furrow field must run 31% more water to achieve the same agronomic result. At pumping costs of $5 to $10 per acre-foot in most aquifer-dependent regions, this efficiency gap translates directly to operating cost.

Drip irrigation, with application efficiencies of 90 to 95%, represents the efficiency frontier for row crops and orchards. Capital costs of $400 to $1,200 per acre make drip a major investment, but payback periods of 5 to 12 years are common for high-value crops. For a 50-acre tomato operation switching from sprinkler (80%) to drip (95%), the efficiency gain reduces gross applied water by 15%, shrinking the pump size and energy footprint correspondingly.

Center pivot systems dominate large-scale grain production because of their scalability, low labor requirement, and reasonable efficiency. A modern variable-rate center pivot with soil moisture sensors can push overall performance above 90%. Furrow irrigation persists in regions with flat topography and heavy soils but is increasingly disadvantaged by low efficiency and salinity accumulation.

Accurate ET scheduling requires local weather data. Several free networks publish daily and weekly ETo by ZIP code or county: the California Irrigation Management Information System (CIMIS) covers California; the Kansas Mesonet, Oklahoma Mesonet, and Texas ET Network serve the Southern Plains; NRCS AgriMet covers the Pacific Northwest and Montana; and the Midwest Climate Center distributes ET data for the Corn Belt. Most Cooperative Extension offices publish weekly crop water use summaries during the growing season.

As a rule of thumb for mid-season peak demand: the US Corn Belt averages ETo of 1.4 to 1.9 in/week in July; the Great Plains runs 1.8 to 2.4 in/week; the Desert Southwest peaks at 2.2 to 2.8 in/week; and the humid Southeast averages 1.2 to 1.6 in/week with higher rainfall offsets. These ranges can anchor your calculator inputs when real-time data is unavailable, but always substitute measured ET when you can — crop water use estimates improve sharply with local data.