Tank Command Area & The Ayacut It Can Feed
Sizes the ayacut
“One accepted answer here saves a hundred people an hour each.
Enter the tank's usable storage, the depth each irrigation needs and the efficiency to get the usable storage and the command area in hectares and acres.
Size your tank command area
Next: plan crops on about 14 ha (34.6 acre); if you need more area, pick a lower-water-depth crop or improve conveyance efficiency above 70%.
Usable = storage × efficiency%. Command area (ha) = usable / (depth_mm × 10), since 1 mm over 1 ha = 10 m³. Real efficiency also loses water to evaporation and conveyance — keep a margin.
Tank command area — key facts
- Command area
- Ayacut a tank can irrigate
- Set by
- Storage, depth, efficiency
- 1 ha-cm
- = 100 m³
- 1 hectare
- ≈ 2.47 acres
- Use
- Usable storage, not gross
- Over-commit
- Leaves the tail-end dry
- Higher efficiency
- Larger command area
- Privacy
- Runs in your browser; nothing uploaded
How much land can your tank water?
An irrigation tank or reservoir can command — that is, irrigate — an area set by three things: its usable storage, the depth of water each irrigation needs, and the conveyance and use efficiency. Convert the stored volume to a depth over an area with the simple rule that 1 ha-cm equals 100 m³, and you have the ayacut the tank can realistically serve. Get this number right and the whole command stays watered.
This tool reports the usable storage and the command area in both hectares and acres, after adjusting for the irrigation depth and efficiency you enter. Over-committing the command area is the classic mistake — it leaves the tail-end fields dry while the head-end takes the lot. Pair it with the Farm Pond, Water Tank Capacity and Seasonal Water Budget tools to plan a tank-fed system that delivers to everyone.
Set a safe ayacut
Match the command area to real storage.
Protect the tail-end
Avoid over-committing and starving it.
Plan in your units
Command area in hectares and acres.
Reward efficiency
See how lined channels stretch storage.
Frequently Asked Questions
What is a tank command area?+
The command area, or ayacut, is the land an irrigation tank or reservoir can actually irrigate. It is set by the usable water in storage, the depth of water each irrigation needs, and how efficiently water is conveyed and used. It is the bridge between a volume of stored water and the area of crop that volume can serve.
How is command area calculated?+
First find the usable storage — the water available for irrigation after dead storage and losses. Then divide it by the total depth of water the crop needs over the season (including conveyance and use losses). The result, converted with 1 ha-cm = 100 m³, is the area the tank can command.
What does 1 ha-cm = 100 m³ mean?+
A hectare-centimetre is one centimetre of water spread over one hectare. One hectare is 10,000 m², and one centimetre is 0.01 m, so 1 ha-cm = 10,000 × 0.01 = 100 m³. It is the natural unit for linking a stored volume in cubic metres to a depth of irrigation over an area.
What is usable storage?+
Usable storage is the part of the tank's volume you can actually draw for irrigation — the gross capacity minus dead storage below the outlet and minus evaporation and seepage losses over the season. Designing on gross capacity overstates the command area, so the tool works from usable storage.
Why does over-committing leave the tail-end dry?+
If the command area is set larger than the usable storage can support, the water runs out before the season ends. Head-end fields near the tank get watered first, and the tail-end fields farthest along the channels are starved. Sizing the ayacut to the real usable storage keeps water reaching everyone.
What depth of water should I use?+
Use the total seasonal water depth the crop needs at the field — its evapotranspiration over the season minus effective rainfall — then adjust for losses with the efficiency. A thirsty long-season crop needs a greater depth and commands less area per unit of storage than a short, low-water crop.
How does efficiency change the answer?+
Conveyance and field-use efficiency is the share of released water that becomes useful crop water; the rest is lost in channels and the field. Lower efficiency means more water must leave the tank for the same crop depth, so the command area shrinks. Lining channels and efficient methods stretch the same storage further.
Does the tool give both hectares and acres?+
Yes. The command area is reported in hectares and acres so you can match it to however your land is recorded. One hectare is about 2.47 acres, so the tool converts for you and you can plan the ayacut in whichever unit your records and neighbours use.
Can I use this for a farm pond?+
Yes. The same logic applies to any stored-water source — a tank, reservoir or farm pond. Enter the pond's usable storage and the crop's seasonal depth and efficiency, and you get the area it can reliably irrigate. See the Farm Pond and Water Tank Capacity tools to estimate the storage itself.
Are the results exact?+
They are a solid planning estimate. Real command area depends on rainfall in the season, actual losses, the cropping pattern and how the tank refills. Treat the output as a guide to set a safe ayacut and avoid over-committing, and adjust as the season's rainfall and demand become clear.
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