Your Farm's Water Budget: Litres In vs. Litres Out, Before You Plant

Ask a farmer in dry country why the crop failed, and you'll hear a dozen honest answers: the rain came late, the borewell dropped, the market crashed, the labour didn't turn up. All true. But underneath most of them sits a quieter, more uncomfortable answer — the water was never going to be enough, and nobody did the sum to find out.

A water budget is that sum. It is the least glamorous document on a farm and the most important. It asks two blunt questions: how many litres will my land receive this year, and how many litres will my crop and the sun demand? If the second number is bigger than the first, no amount of hard work closes the gap. You are farming a deficit, and a deficit always gets paid — usually by the crop, sometimes by the borewell, occasionally by the farm itself.

This piece teaches you to draw that budget for a dry, monsoon-dependent plot. It's beginner-leaning on purpose. You don't need hydrology; you need a pencil, honest numbers, and the willingness to hear a "no" from your own land before the season does the telling.

The one idea: your farm is a bank account, in litres

Keep this and you have the whole article: water is money, and your farm is an account that fills a few times a year and drains every single day.

Inflows (deposits): rain that soaks in or is harvested, borewell or well yield, any canal or tanker water.
Outflows (withdrawals): what the crop transpires, what the soil surface and open water evaporate, what seeps or runs off unused.

A budget just lines them up. If withdrawals exceed deposits over the season, you overdraw — and unlike a bank, your land charges the penalty in wilted plants.

Counting the litres coming in

Start with rain, because it's the biggest and freest deposit — and the one most people count wrong.

Rain on your land. One millimetre of rain over one square metre is one litre. So over one acre (about 4,047 m²), a single millimetre of rain drops roughly 4,047 litres. If your dependable seasonal rainfall is, say, 550 mm, that's about 22 lakh litres per acre falling on your land across the monsoon. ^†

But — and this is the catch beginners miss — you don't get to keep all of it. Some runs off before it soaks in. Some evaporates off hot bare soil within hours. The fraction that actually enters your root zone and stays useful is far less than what falls. On bare, crusted red soil in a hard downpour, a large share can run straight off. Good soil cover, bunding and mulch tilt that fraction back in your favour — which is exactly why they're water infrastructure, not just soil practice.

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"Effective rainfall" is the honest number. Irrigation planners don't budget total rainfall; they budget effective rainfall — the part crops can actually use after runoff and deep drainage. A rough planning rule sometimes used is that only a portion of monsoon rain is effective, and it varies with rainfall intensity and soil. FAO's standard rule of thumb (for slopes under ~4–5%) treats effective rainfall as Pe = 0.8·P − 25 for months above 75 mm and Pe = 0.6·P − 10 below it — i.e. only about 60–80% of a month's rain, minus a fixed loss, actually reaches the root zone [FAO, Irrigation Water Management Training Manual 3 — Irrigation Water Needs, Brouwer & Heibloem, 1986, §4.2]. Verify the fraction for your own soil and rainfall intensity rather than treating any single number as universal.

Borewell and well. Deposit the dependable yield, not the best-day yield. A borewell that gives 2 inches of water for two hours in July can give a trickle in April — and April is when you need it. Budget the dry-month yield, because that's when the account is closest to empty. If you don't know it, measure it: time how long it takes to fill a known drum. ^†

Counting the litres going out

Now the withdrawals — and here the sun is a bigger spender than most farmers credit.

Crop water demand. Every crop transpires a broadly known quantity of water over its season, driven by climate and crop stage. The standard method — reference evapotranspiration (ET₀) multiplied by a crop coefficient (Kc) — is a real, well-documented approach [FAO Irrigation & Drainage Paper 56, Allen et al., 1998 — VERIFY the exact crop-coefficient values for your crop and stage before relying on them]. In practical terms, a season's crop water demand in dry country commonly runs to several lakh litres per acre — and thirsty crops like paddy or sugarcane can demand multiples of what millets or pulses do.

Evaporation from soil and open water. In hot, dry pre-monsoon months, open water can lose on the order of 6–8 mm per day off its surface ^†. Bare, unmulched soil bleeds moisture the same way. Every day of exposed soil is a silent withdrawal you never authorised.

Seepage and runoff losses. Water that drains below the root zone or runs off unharvested is a withdrawal you get nothing for. It isn't wasted in nature — it may recharge the aquifer — but from the crop's budget, it's spent.

A farm doesn't run out of water on the day the crop wilts. It runs out on the day, months earlier, when nobody did the arithmetic. — grOrganic field notes

Putting it together: the balance line

Lay the two columns side by side for one acre, one season. This is deliberately rough — a planning sketch, not an engineering model.

Litres IN (deposits)	Litres OUT (withdrawals)
Effective rain into root zone	Crop seasonal water demand (ET₀ × Kc)
Harvested runoff (pond/tank you can pump)	Soil-surface evaporation
Dependable borewell / well yield	Open-water evaporation (ponds, channels)
Canal / tanker (if any)	Unrecoverable seepage & runoff
= TOTAL IN	= TOTAL OUT

If TOTAL IN minus TOTAL OUT is comfortably positive, you have a buffer for the year the rain fails. If it's thin or negative, you have three honest levers — and no fourth:

Increase reliable inflow — harvest more runoff, deepen storage, add recharge. (Slow and capital-heavy.)
Cut outflow — mulch to kill soil evaporation, drip to cut waste, cover open water. (Fast, cheapest per litre saved.)
Choose a thriftier crop — match demand to supply instead of the reverse. (Free, and the most powerful.)

Going deeper (for the practitioner)

A single seasonal total hides the real risk, which is timing. A budget that balances over the whole monsoon can still fail if a three-week dry spell lands during flowering — the crop's most water-sensitive stage. The serious version of this exercise is a stage-wise or monthly water balance: inflows and outflows month by month, matched against crop-stage demand, so you can see the pinch point before it arrives. Build the annual sketch first; graduate to the monthly ledger once the habit sticks. ^†

The point of the whole exercise is not precision — your numbers will be rough, and that's fine. The point is direction: to know, before you spend a rupee on seed, whether you are farming a surplus or an overdraft.

Sources & to-verify

Confirmed: - ET₀ × Kc crop-water method — FAO Irrigation & Drainage Paper 56, Crop evapotranspiration: Guidelines for computing crop water requirements (Allen, Pereira, Raes & Smith, 1998); the reference-ET times crop-coefficient approach is the documented standard. https://www.fao.org/4/x0490e/x0490e00.htm (specific Kc values remain crop-/stage-specific — see ^† below.) - Effective-rainfall method — FAO Irrigation Water Management Training Manual 3: Irrigation Water Needs (Brouwer & Heibloem, 1986), §4.2: Pe = 0.8·P − 25 (P > 75 mm/month) and Pe = 0.6·P − 10 (P < 75 mm/month) for slopes under ~4–5%. https://www.fao.org/4/s2022e/s2022e08.htm - Relative seasonal crop water demand (paddy/sugarcane vs. millets/pulses) — FAO Training Manual 3, Table 14 (indicative crop water needs): sorghum/millet 450–650 mm, bean/pea 300–500 mm, rice 450–700 mm, sugarcane 1,500–2,500 mm — a several-fold gap. https://www.fao.org/4/s2022e/s2022e07.htm

Cited / referenced (confirm before publishing): - Unit fact: 1 mm rain over 1 m² = 1 litre; 1 acre ≈ 4,047 m² — standard, safe.

^† — must be fact-checked with a real source before publishing: 1. Taluk/district 75% dependable seasonal rainfall — the ~550 mm is a placeholder. Source: IMD or state rainfall agency. 2. Crop coefficient (Kc) values for the reader's specific crop and growth stage — FAO-56 or state agri-university tables. 3. Open-water / pan evaporation (6–8 mm/day pre-monsoon) and the ~0.7 pan-to-open-water factor — verify against IMD pan-evaporation data for the district. 4. Crop-stage water sensitivity (flowering/grain-fill most sensitive) — cite a crop-physiology or FAO-56 reference. 5. Your own borewell dry-season yield — measure on-farm; never budget the best-day figure.

Key takeaways

A water budget is litres-in vs. litres-out, done before you plant. Most "the crop failed" stories are an uncalculated deficit wearing a different costume.
Count effective rainfall, not total rainfall, and dependable dry-season yield, not best-day yield — the honest numbers are lower and they're the ones that bind.
The sun is a major spender. Evaporation off bare soil and open water is a real, daily withdrawal; mulch and cover are water infrastructure.
When the budget is tight, cutting outflow (mulch, drip, cover) is cheaper per litre than finding more inflow — and choosing a thriftier crop is cheaper still.
Match the crop to the water, never the water to the crop.

Your next step — draw the two-column budget below for one acre before you finalise this season's crop. Change any input and the balance updates live:

Enter your rainfall, borewell yield, area and crop demand; the balance line recalculates instantly.

Region/season caveat: These figures and fractions are planning brackets for dry, semi-arid, monsoon-dependent country and its pre-monsoon planning window; your own dependable rainfall, soil, borewell yield and crop coefficients will differ — verify every bracketed number locally before you sow.