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Home > Agronomic Practices > Irrigation Water Management
Irrigation Water Management

Sugarcane being a long duration crop producing huge amounts of biomass is classed among those plants having a high water requirement and yet it is drought tolerant. It is mostly grown as an irrigated crop. The plant crop season being 12-18 months in India, 13-14 months in Iran, 16 months in Mauritius, 13-19 months in Jamaica, 15 months in Queensland (Australia) and 20 - 24 months in Hawaii.

 

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Moisture Extraction Pattern

Most root biomass for sugarcane is found close to the surface and then declines approximately exponentially with depth. Typically, approximately 50% of root biomass occurs in the top 20 cm of soil and 85% in the top 60 cm. The percentage of roots in the 0-30 cm horizon was 48-68%; from 30 to 60 cm, 16 - 18%; 60 to 90 cm, 3-12%; 90 to 120cm, 4-7%; 120 to 150 cm, 1-7%; and 150 to 180 cm, 0-4%. Thus the moisture extraction pattern from different soil layers follows the root biomass distribution.

 

Root growth responds strongly to the soil environment, creating plasticity in the form and size of the root system. The size and distribution of the root system is strongly affected by the distribution and availability of soil water, causing differences in the capacity of crops to exploit deeper soil reserves.

 

Root distribution of sugarcane crop raised on loamy soil irrigated at 7, 14 and 21 days interval. Roots of a 12-month old plant crop were more deeply distributed under less frequent irrigation presumably in response to drying of the surface. Deeper rooting reduces the vulnerability of crops to soil water deficits by providing increased capacity for uptake of deep reserves of soil water. It also aids in reducing lodging. Hence, drip irrigated cane should be scheduled irrigations at less frequency during the initial 2 to 3 months period to promote deeper rooting. (In the graph above: Distribution of root riomass with depth for sugarcane irrigated at frequencies of 7 days, 14 days and 21 days (Baran et. al., 1974)

 

Nutrient supply has also been shown to similarly affect the rooting patterns. High soil strength causes slower root growth with reduced branching and thickened roots. High water markedly affects root distribution, with a majority of studies showing that rooting ceases within approximately 0.1 m of static water tables.

 

Restricted root growth above shallow water tables does not necessarily reduce crop growth, as capillary rise can supply the crop with water and instances of water uptake from within the saturated zone have been observed.

 

A risk of water stress does result from the lack of root penetration in soils with high water tables if ground water height falls rapidly, leaving the root system restricted to dry soil.

 

Physiological Characteristics to be Considered for Efficient Water Managment

  • A liberal water supply reduces the cane yield and/or sugar yield, while mild water stress enhances the yield
  • Excessive watering at tillering should be avoided since it coincides with active root development and hinders nutrient uptake due to poor O2 diffusion
  • Length of the cane determines the sink available for sugar storage since there is no secondary thickening of the stem in sugarcane
  • A drying off or cut out period of 4-6 weeks prior to harvest ensures an optimum sugar yield
  • Reduction of water during the ripeness to flower stage helps to control flowering or arrows

 

Field Irrigation Schedule

The goal of an efficient irrigation scheduling programme is to "provide knowledge on correct time and optimum quantity of water application to optimize crop yields with maximum water use efficiency and at the same time ensure minimum damage to the soil". Thus,

  • Irrigation scheduling is the decision of when and how much water to apply to a cropped field.
  • Its purpose is to maximize irrigation efficiencies by applying the exact amount of water needed to replenish the soil moisture to the desired level.
  • Make efficient use of water and energy.

 

Therefore, irrigation scheduling for sugarcane involves precise estimation of depth of water to apply at each irrigation, and the interval between irrigation, for each soil-plant-climatic condition. With drip irrigation, intervals of irrigation are usually daily irrespective of the evaporative demand of the atmosphere.

 

The crop evapotranspiration under standard conditions, denoted as ETc, is the evapotranspiration from disease free, well-fertilized sugarcane crop, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic condition. The amount of water required to compensate the evapotranspiration loss from the cropped field is defined as crop water requirement.

 

Although the values for crop evapotranspiration and crop water requirement are identical, crop water requirement refers to the amount of water that needs to be supplied, while crop evapotranspiration refers to the amount of water that is lost in evaporation + transpiration.

 

The irrigation water basically represents the difference between the crop water requirement and effective precipitation. The irrigation water requirement also includes additional water for leaching of salts and to compensate for non-uniformity of water application.

 

Adequate soil moisture throughout the crop-growing season is important for obtaining maximum yields because vegetative growth including cane growth is directly proportional to the water transpired. Depending on the agro-ecological conditions, cultivation practices adopted and crop cycle (12-24 months) seasonal water requirements of sugarcane are about 1300mm to 2500mm distributed throughout the growing season (Table 10). The amounts of water required to produce 1.0-kg cane, dry matter and sugar are 50 - 60, 135-150 and 1000-2000g, respectively. The transpiration coefficient of sugarcane is around 400. This means 400 m3 of water is required to produce one ton of dry matter.

 

Table 10. Sugarcane Water Requirements in Various Countries

 

S.No.

Country

Water requirement (mm)

1.

Australia

1522 (Drip)

2.

Burundi, Central Africa

1327 to 2017 (Furrow)

3.

Cuba

1681 to 2133 (Plant)

4.

Hawaii

2000 to 2400 (24 months)

5

Jamaica

1387

6

Mauritius

1670 (Drip)

7

Philippines

2451 (Furrow)

8

Pongala, South Africa

1555

9

Puerto Rico

1752

10

South Africa

1670

11

Subtropical India

1800 (Furrow)

12

Taiwan

1500 to 2200 (Furrow)

13

Tropical India

2000 to 2400 (Furrow)

14

Venezuela

2420 (Furrow)

15

Thailand

2600 (Furrow)

Source: Several

 

The Calculation Procedure for Crop Evapotranspiration, ETc, Consists of: 

 

Calculation of Reference Crop Evapotranspiration (ETo)

Collect available climatic data; based on meteorological data available, select prediction method viz., either Penman-Monteith or Pan evaporation method to calculate ETo. Compute for necessary periods considering the growth subperiods of the crop in question.

 

Construction of Crop Coefficient (Kc) Curve

Determine time of planting, identify the crop growth subperiods, determining their lengths; Select Kc for a given crop growth sub-period under prevailing climatic conditions. Construct the crop coefficient curve (allowing one to determine Kc values for any period during the growing period); and

 

Calculation of ETc under standard conditions as a product of ETo and Kc.
The reference crop evapotranspiration (ETo) estimated based on Penman-Monteith or Adjusted USWB Class A Pan evaporation method reflects the evaporative demand of the atmosphere for the location in question.

In the picture: USWB Class a Pan Evaporimeter for Estimating ETo

 

While the crop factor (a dimensionless ratio) reflects the crop characteristics and indicates the combined loss of water from a sugarcane field both by transpiration and soil evaporation (Crop ETc) relative to ETo over the same period.

 

Several workers have worked out estimates of crop factors experimentally for different crop growth stages of sugarcane. The daily requirement in millimeters is converted to the equivalent volumetric quantity for the area under drip (1 mm = 10 m3/ha).

 

A field irrigation schedule prepared based on above approach for irrigating sugarcane grown in Tropical region of India is presented in the picture for field application.

 

While the water requirement during different crop-growth subperiods and cumulative ETc is depicted in the picture here. Such type of irrigation scheduling programmes can be prepared for each location (depending up on the climatic data availability) in real time operation.

 

Water Supply and Cane Yield

Frequency and depth of irrigation should vary with growth periods of the cane. The relationship between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growth sub-periods is shown in the picture. During the initial germination, field emergence and establishment of young seedlings the crop requires less water, hence light and frequent irrigation water applications are preferred.

 

The water supply must be just sufficient to keep the soil moist with adequate aeration. If the soil is allowed due to infrequent and less water application, the germinating buds get desiccated leading to a lower and delayed germination.

 

On the other hand over irrigation leads to bud rotting due to lack of aeration, fungal attack and soil reducing soil temperature. Thus both under and over irrigation are detrimental for germination, resulting in low stalk population per unit area.

 

During the early vegetative period (formative) the tillering is in direct proportion to the water application. An early flush of tillers is ideal because this furnishes shoots of approximately same age. Any water shortage during tillering phase would reduce tiller production; increase tiller mortality and ultimately the stalk population-an important yield component.

 

However, excess irrigation during tillering phase is harmful particularly in heavy soils, since it coincides with active root development, which may be hampered by anaerobic condition created in the soil as a result of over irrigation.

 

The yield formation or grand growth period is the most critical period for moisture supply in sugarcane. This is because the actual cane yield build-up or stalk growth takes place in this period. The production and elongation of inter-nodes, leaf production on the stalk and its expansion, girth improvement, ultimately the stalk weight takes place in this period. It is also the period for production of sugar storage tissues. Therefore, crop reaches its peak water requirement in this stage. With adequate water supply to maintain a sheath moisture content of 84-85% in the leaf sheaths, 3,4,5 and 6 from the top during this period of active growth produces longest inter-nodes with more girth (thick cane) and the total cane weight is greater.

 

On the other hand water deficits in yield formation period reduce stalk elongation rate due to shortening of inter-nodes resulting in less cane weight and the effect is well marked on yield at harvest. A severe water deficit during the later part of the grand growth period forces the crop to ripen. In many an areas in India the tillering/early yield formation period coincides with hot weather period (March - June).

 

The evaporative demand of the atmosphere is very high during hot weather period vis-√ -vis the crop water needs. Therefore management of available water supplies to meet the peak water requirement is very crucial to realize optimum yield potential.

 

In ripening period the a restricted water supply or mild water deficits (sheath moisture content of 74-76%) is necessary to bring the crop to maturity by reducing the rate of vegetative growth, dehydrating the cane and forcing the conversion of total sugars to recoverable sucrose. With the check of vegetative growth, the ratio between dry matter stored as sucrose and that used for new growth increases.

 

On the other hand plentiful supply of water leads continued vegetative growth thus affecting sugar accumulation process. However, when the plant is too seriously deprived of water, it would disrupt the plant metabolism and loss of sugar content can be greater than sugar formation. An important consideration is that soil should not be allowed to crack, as it would cause root pruning and damage the root system.

 

When the crop is in the ripening period, a farmer may also have a just planted crop on his farm in most situations. Therefore, the tendency of the farmer will be to provide sufficient water to the new (young) crop and neglect the grown up crop that is to be harvested. This situation is particularly true under limited water availability situations. If the grown up crop is not irrigated as required it experiences severe water deficits and there could be cane breakage, pith formation, significant reduction in cane weight, increase in fibre content and deterioration in juice quality.

 

The situation is further aggravated if the harvesting is delayed. Thus both the farmer and the factory would suffer. Therefore, even for the grown up crop, reasonable amount of water with restricted supply is necessary to obtain good cane yield.

 

Sheath Moisture Content

The sheath moisture or relative water content determined by crop logging technique is presented in Fig. It has long been used to control water application to commercial sugarcane crop, more particularly during ripening phase, when gradual increase in water stress is used to stimulate sugar storage in the stem.

 

A ripening log is used to compare measured and desired sheath water content during approximately 12 to 24 weeks (depending upon the crop duration) prior to harvest.

 

Sheath water content is measured on a periodic basis, and irrigation intervals and amounts are varied to produce a gradual decline of sheath water content, from about 83% at the beginning of ripening to about 75% at harvest.

 

In Hawaii and Taiwan sheath water content has been found to be a good indicator of stem sugar content. Similar methods involving other tissues are in use in Mexico, South Africa, India and Zimbabwe.

  

Irrigation water is often limited and costly input. Therefore determination of optimum amount of water over a crop period to achieve higher water use efficiency assumes significance.

 

Several experiments conducted world over have indicated that the relationship between cane yield and seasonal crop water use under a given climatic condition was linear. 

 

When irrigation plus rainfall is greater than the crop water requirement, anaerobic soil conditions or N losses may reduce crop growth rates and cane yield.

In the graphs / pictures above:

  • Cane yield response to water deficits (Doorenbos and Kassam, 1979 )
  • Optimum Sheath Moisture content indices for sugarcane (Lakshmikantham, 1983)
  • Cane Yield Response to seasonal evapotranspiration [Thompson (1976) & Jones (1980)]

 


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