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# Estimating Irrigation Demand (Part - 1) Civil Engineering (CE) Notes | EduRev

## Civil Engineering (CE) : Estimating Irrigation Demand (Part - 1) Civil Engineering (CE) Notes | EduRev

The document Estimating Irrigation Demand (Part - 1) Civil Engineering (CE) Notes | EduRev is a part of Civil Engineering (CE) category.
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Introduction

A plot of land growing a certain crop or a combination of crops has to be supplied with water from time to time. Primarily, the plot or field is expected to receive water from rain falling on the land surface. But, as we know, the distribution of rain is rather uncertain both in time and space. Also some of the rain as in a light shower does not reach the ground as it may be intercepted by the leaves of the plant during a heavy downpour; much of the water might flow away as surface runoff. Hence, only a certain amount of falling rain may be effective in raising the soil moisture that is actually useful for plant growth. Hence, for proper crop growth, the effective rain has to be supplemented by artificially applying water to the field by irrigation.

If the area of the field is small, water may be supplied from the local ground water source. If the field is large, supplemented irrigation water may be obtained from a local surface water source, like a river, if one is available nearby. The work of a water resources engineer therefore would be to design a suitable source for irrigation after knowing the demand of water from field data. In this lesson, we proceed on to find out the methods by which estimation may be made for irrigation water demand.

Crop water requirement

It is essential to know the water requirement of a crop which is the total quantity of water required from its sowing time up to harvest. Naturally different crops may have different water requirements at different places of the same country, depending upon the climate, type of soil, method of cultivation, effective rain etc.

The total water required for crop growth is not uniformly distributed over its entire life span which is also called crop period. Actually, the watering stops same time before harvest and the time duration from the first irrigation during sowing up to the last before harvest is called base period. Though crop period is slightly more than the base period, they do not differ from practical purposes. Figure 1

indicates the relative usage of water for a typical crop during its entire growth period.

FIGURE 1. Variation in the requirement of water for paddy with stage of growth (Image courtesy: Food and Agriculture Organisation, FAO)

Sometimes, in the initial stages before the crop is sown, the land is very dry. In such cases, the soil is moistened with water as t helps in sowing the crops. This is known as paleo irrigation. A term kor watering is used to describe the watering given to a crop when the plants are still young. It is usually the maximum single watering required, and other waterings are done at usual intervals.

The total depth of water required to raise a crop over a unit area of land is usually called delta. Some typical values of delta for common crops in some regions of India are as follows:

Rice

• 1000mm to 1500mm for heavy soils or high water table
• 1500mm to 2000mm for medium soils
• 2000 to 2500 for light soils or deep water table
• 1600mm for upland conditions

Wheat

• 250mm to 400mm in northern India
• 500mm to 600mm in Central India
• Barley: 4 50mm

Maize

• 100mm during rainy season
• 500mm during winter season
• 900mm during summer season
• Cotton: 400 – 500mm

Sugarcane

• 1400mm to 1500mm in Bihar
• 1600mm to 1700mm in Andhra Pradesh
• 1700mm to 1800mm in Punjab
• 2800mm to 3000mm in Maharashtra

Duty of water

The term duty means the area of land that can be irrigated with unit volume of irrigation water. Quantitatively, duty is defined as the area of land expressed in hectares that can be irrigated with unit discharge, that is, 1 cumec flowing throughout the base period, expressed in days.

Imagine a field growing a single crop having a base period B days and a Delta ∆ mm which is being supplied by a source located at the head (uppermost point) of the field, as shown in Figures 2 and 3.

FIGURE 2. Border irrigation method of applying water at the head of a field (Image courtesy: Food and Agriculture Organisation, FAO)

FIGURE 3. Furow irrigation method of applying water to a field (Image courtesy: Food and Agriculture Organisation, FAO)

The water being supplied may be through the diversion of river water through a canal, or it could be using ground water by pumping (Figure 4).

If the water supplied is just enough to raise the crop within D hectares of the field, then a relationship may be found out amongst all the variables as:

Volume of water supplied = B*60*60*24 m3

Area of crop irrigated = D*104 m2

Volume of water supplied per unit area  =86400/10000D = 8.64B/D

Hence, knowing two of the three variables B, D and ∆ the third party may be found out.

The duty of irrigation water depends upon a number of factors; some of the important ones are as follows:

• Type of crop: As different crops require different amount of water for maturity, duties are also required. The duty would vary inversely as the water requirement of crop.
• Climate season and type of soil: Some water applied to the field is expected to be lost through evaporation and deep percolation. Evaporation loss has a direct bearing on the prevalent climate and percolation may be during drier seasons when the water table is low and soil is also dry. Percolation loss would be more for sandy soils than silty or clayey soils.
• Efficiency of cultivation methods: If the tillage and methods of water application are faulty and less efficient, then the amount of water actually reaching the plant roots would be less. Hence, for proper crop growth more water would be required than an equivalent efficient system. Also, if the water is conveyed over long distances through field channels before being finally applied to the field, then also the duty will rise due to the losses taking place in the channels

Crop growing seasons in India

Each crop has its own sowing and harvesting seasons and it is important to have a knowledge of this which may help to decide the total water demand in a field having mixed crops.

In India, the northern and north eastern regions have two distinct cropping seasons. The first coinciding mostly with the South western monsoon is called kharif , which spans mostly from July to October. The other, called rabi, spans generally over October to March. The summer season crops are planted sometime between April and June. In southern part of India, there is no such distinct season, but each region has its own classification of seasons.

Generally, the kharif is characterized by a gradual fall in temperature, more numerous cloudy days, low intensity, high relative humidity and cyclonic weather. During Rabi, there is a gradual rise in temperature, bright sunshine, near absence of cloud days, and a lower relative humidity.

The following table indicates some the regional cropping calendars in India.

 State Season Local name Growing month Andhra Pradesh Kharif Serva or Abi July - December Rabi Dalwa or Tabi December - April Summer In limited areas March/April    - June Assam Pre-monsoon Ahu Mar/April- June/july - Sali June/July- Nov/Dec - Boro Nov - May Bihar Summer - March - July/Aug Autumn - May/June- Sept/Oct Winter - June - Nov/Dec Gujarat Kharif Chomasu Dangar June/July-Oct/Nov - Unala Dangar Dec - June Haryana Kharif - May/June- Sept/Oct Himachal Pradesh Kharif - June/July- Sept/Oct Jammu & Kashmir Jammu: June-Nov Kashmir:    Last week of April -October Karnataka Kharif - June - Dec Summer - Jan-May/June Kerala first crop Virippu April-May/Sept- Oct Second crop Mundakan Sept-Oct/Dec-Jan Third crop Punja Dec/Jan-Mar/April Madhya Pradesh Kharif - June/July-Dec Maharashtra Kharif - June/July-Dec Manipur Kharif - Mar/June- Sept/Oct Meghalaya Kharif - May/June- Aug/Sept Rabi - Nagaland Kharif - May/June- Nov/Dec Rabi - Feb - May Orissa - Sarad June-Dec - Dalua Dec-April - Beali (short Duration) April/May -Sept (Only in uplands) Punjab Kharif - May - Nov Rajasthan Kharif - June/July-Sept/oct Tamil Nadu - Navarai Jan-April - Sornavari April - July - Kar or Kuruvai June - August - Samba June/July- Nov/Dec - Thaladi or Pishanam Sept/Oct- Feb/March Uttar Pradesh Kharif - June - Oct West Bengal Pre-Kharif Aus April-Sept Kharif Aman June-Dec Summer Boro End Nov-Mid June
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