When and how much water to irrigate?

Basic irrigation computations are need to assure the success in your farms. Irrigation is the artificial addition of water in the farm. However, how do you know you need to irrigate? Irrigation is a tedious task especially when you don’t have any experience. Almost all farmers only detects water stress the leaves are already not turgid. It is their signal to irrigate their crops. Is this good indication? Not really, there is a science in irrigating your crops let me discuss these things to you. Use the tables below to solve for basic irrigation computations.


Table 1. Suggested maximum water application rate for sprinkler for average soil, slope and cultural (mm/hr) conditions.

Soil texture                                                                                         Maximum H₂O Application Rate

Profile condition                                                                               0% slope                    1% slope


Light sandy loams                                                                            w/cover     bare       w/cover        bare

Uniform more compact                                                                    43            25               25                 15

To 2m


Light  sandy loams

Over more compact                                                                            30           18               18                  10

Sub soil


Silt loam Uniform                                                                                25            13              15                   8

In texture to 2m


Silt loam over                                                                                         15              8                 10                3

More compact subsoils


Heavy textured clay                                                                             5                       3              3                   2

or clay loam      


This table indicates the maximum application of water in your farm. The maximum application depends on the soil texture. since, every soil has different characteristics when comes to dealing with water. This is to avoid saturation in field. Saturation is bad for your plants because some plants cannot tolerate so much water. It can also add up to your expenses which is very bad. Let take for example, sands can carry less water because of it is more porous than other soils. It is good? Depends on the crop that you will be planting. There are crops which love a certain soil texture.


Table 2. Moisture holding capacities

Sandy loam                         –              130mm/m depth or 13% by volume

Sit loam                                –              150mm/m depth or 15% by volume

Clay loam                             –              170mm/m depth or 17% by volume

As we go deeper in irrigation, we should understand in every layer of the soil there is a certain moisture capacity. This moisture capacity is determine its soil texture again. Soil is like a sponge that can carry water and air.


Table 3. Peak rate of moisture use (mm/d)

Crop                      Root Depth (m)                Cool       Climate moderate                      Hot


Beans                    0.3-0.9                                  3                            4                                            6

Corn                      0.6-1.2                                  5                           6                                            8

Pasture                0.5-0.8                                  5                            6                                            8

Potatoes              0.3-0.6                                  4                            5                                            6

The peak rate of moisture is the maximum amount of water that the crop will use per day. It is the summation of all parameters in evaporation and transpiration. Every crop has its different root depth and consumptive use in every temperature.


Irrigation Example

A sprinkler system is to be designed to irrigate 10 hectares of pasture in silt loam soil over a compact subsoil. The field has 0% in slope which means it is flat. Solve the limiting rate of application, the irrigation period, net depth of water application, and required system capacity in hectares per day. Compute the basic irrigation computations?


Area = 10 hectares

Vegetation = pastured

Soil = silt loam over a compact sub soil



  1. Limiting rate of application
  2. Net depth of Water Application
  3. Irrigation Period
  4.  System Capacity in hectares per day


1.) Limiting rate of application = 15 mm/hr

Please check Table 1 for more details.


2.) Net depth of water application

Please check Table 2 for more details.


Silt loam = 15% volume or 150 mm/m depth of root  zone.

Depth of root zone = 0.65


I used 0.65 as the average as the value for root zone. Please check Table 3 for more details. However, this can be change depending on the root zone of your crop.


Total Available Moisture = ( 150 mm/m) (0.65m) = 97.5 mm

Net depth of application = (97.5mm)(0.45) = 43.88 mm

Moisture retained in the soil = ( 97.5 mm) (0.55) =53.625 mm


There must be a retained water in the your area at least of 55%. When water goes down to this level or below this level irrigation is a must. Therefore, the water depth that should be replenish is 43.88 mm. Let us take in to consideration the system losses. For example, you have the efficiency of your irrigation system is 75%. What should be the amount of water to pump out of the water source?


Total amount to be pumped = 43.88mm/0.75

= 58.50 mm


3.) Irrigation period

Peak rate use of pasture = 6mm/day

Irrigation period = net depth application / peak rate

= 43.88mm/(6mm/day)

= 7.31 days


4.) Capacity of the system, hectare per day


Capacity = 10 hectares/ 7.31 days =1.36 hectares per day

Irrigation Engineers, we salute you!


Irrigation engineers have one of the most critical jobs in agriculture especially  in large area of plantations. Plants have life and should be take care properly. These plants should not be over watered to avoid water stress. Water stress kills plants and has detrimental effect on soil. It increases run off in your plantation. Thus, soil degradation will cause more problems in your area. Irrigation engineers which are agricultural engineers should be a good soil and water conservationist. They must assure that there is an optimal amount of water in farms. These Basic Irrigation Computations are need to assure the a good amount of production. Remember, even humans need water how much more of plants who are all days in the field.