Scheduling an irrigation
irrigation schedulingwhen to irrigateavailable water capacitycrop water useset run time
Scheduling irrigation: what the soil holds, the depletion you allow, crop water use, the interval between sets and the hours to run.
Available Water Capacity
The depth of water a soil can hold that a crop can actually use — the gap between field capacity and the permanent wilting point, multiplied by the depth of rooting that reaches it.
Readily Available Water from MAD
The depth of water a manager is willing to let the crop draw down before irrigating — the available water capacity multiplied by the management allowable depletion fraction. This, not the full reserve, is the size of an irrigation.
Crop Evapotranspiration
The water a specific crop uses on a given day, from a reference evapotranspiration that describes the weather and a crop coefficient that describes the canopy. The FAO-56 two-step method, which separates what the atmosphere demands from what the plant does about it.
Net Irrigation Requirement
The depth of water irrigation must supply over a period: everything the crop used, less the rainfall that actually reached the root zone. The word doing the work is effective — rain that ran off or drained past the roots is not in this equation.
Irrigation Interval
How many days a crop can go between irrigations: the depth of water it is allowed to use, divided by the depth it uses each day. The whole of irrigation scheduling in one division.
Irrigation Set Run Time
How long a set must run to lay down a wanted depth of water over a known area at a known flow rate — depth times area is a volume, and a volume divided by a flow rate is a time.
Irrigation System Capacity
The flow an irrigation system must deliver to keep up with peak crop water use, once application efficiency and the hours a day the system can actually run are accounted for. The sizing calculation behind every pump, well and mainline on the farm.
How they fit together
Irrigation scheduling is a bank account. The soil is the account, rain and irrigation are deposits, the crop makes a withdrawal every day, and the job is to work out when the balance gets low enough to matter. Available water capacity sizes the account: field capacity minus permanent wilting point, times rooting depth. Both of those terms carry the trap. AWC is not total water in the soil — it excludes the water held so tightly the roots cannot have it, which is why a heavy clay holding a great deal of water can still be a poor reservoir for a crop. And rooting depth is the crop's, not the soil profile's: the same field has three times the reservoir for a mature maize crop that it has for a seedling.
Readily available water applies management allowable depletion, and MAD is a decision rather than a measurement. Typically 50% for a field crop, and much tighter, 30% or less, for shallow-rooted or high-value horticulture where any stress costs quality. Choosing MAD is choosing your risk. Crop evapotranspiration is the daily withdrawal, Kc times a reference ET0, and the crop coefficient moves right through the season — around 0.3 at emergence, up past 1.15 at full canopy, back down at senescence. Scheduling with a mid-season Kc in June is how a crop gets stressed in a wet month. Net irrigation requirement subtracts effective rainfall, and the word effective is doing work there too: a 20 mm storm on a dry cracked soil may put 12 mm in the root zone, and a 40 mm storm on a wet one may put in almost nothing.
The last three are the operating decisions. Irrigation interval is RAW divided by daily ETc, which is the number of days before the account hits the limit you set, and run time is the depth you want, times the area, divided by the flow you have. Run these against each other and the real constraint appears. System capacity is the check that decides whether the schedule is achievable at all: sized against peak ETc rather than average, with application efficiency and the fraction of the day the system can actually run. This is the formula that catches the common failure, which is not a system that cannot water the field but one that cannot get back around to the first set before it needs water again. If the interval is shorter than the time it takes to complete a full rotation, the schedule is arithmetic rather than a plan, and the answer is more capacity or less area.