Keywords

1 Introduction

India has only 2.4% of land surface and 4% of fresh water surface in the world. As significant surface water scarcity is increasing day by day, so, increasing water demand for irrigation becomes national importance for any country for utilizing water resources, conservation of water for modern irrigation approaches and management policy and practice [1]. India is constantly facing drought situation and is in urgent need of water to enhance the irrigation operation for different crops. The rainfall distribution was insufficient and irregular for the year 2005 of Kurnool District. A very less amount of rainfall was occurring during post-monsoon season (i.e. December–February) and most of the rabi crops and perennial crops had faced a significant water stress at the root zone during this year affecting the productivity. Crop water requirement of various crops and climatic water balance was evaluated in different Agro-Ecological Unit’s (AEU’s) by using evapotranspiration as well as effective rainfall data in Palakkad District, Kerala [2]. Water requirement of crop and irrigation scheduling was studied for some major crops in Dhi-Qar province, Southern Iraq by using CROPWAT 8.0 [3]. Guidelines for the evaluation of reference crop evapotranspiration (ETo), crop evapotranspiration under standard and non-standard conditions, irrigation water balance and yield reduction under some adverse field conditions, are illustrated in ‘FAO - Irrigation and drainage Paper 56’ [4]. To enhance the estimation of needed yield under significant water scarcity scenario, ‘AQUACROP’ model is used widely. Canopy growth profile, canopy senescence and stomatal closure, and harvestable index are some of the essential crop characteristics [5]. An empirical model has been established to build a relationship between crop production and evapotranspiration in AQUACROP by using the linear dependency of biomass with transpiration within a range of water deficit [6]. Crop yield prediction models have a significant contribution in estimating food demand with existing population under available climatic and environmental conditions [7]. AQUACROP software demands a limited set of input variables which makes this software both simple and effective as compared to models [8].

The objectives of this present study are lying (i) to estimate the reference crop evapotranspiration and effective rainfall by CROPWAT 8.0; (ii) to show the contribution of effective rainfall in CWR and to develop irrigation scheduling of sugarcane; (iii) to study the crop characteristics for sugarcane under limited set conditions by AQUACROP 6.1 model.

2 Materials and Methodology

2.1 Study Area

Monthly climatic and meteorological data for 2005 were taken at Kurnool district, Andhra Pradesh. This district is located between 14°54′ N to 16°18′N latitude and 76°58′E to 79°34′E longitude and altitude of 94.5 m from m.s.l. The mean monthly temperature is lying between 21.5 °C and 36 °C during winter and summer seasons respectively. Some major crops are cultivated here i.e. cotton, sunflower, groundnut, rice, onion, sorghum etc. under tropical climate. Net irrigated area of Kurnool district is 1757 km2, gross irrigated area is 2126 km2 and rainfed area is 6858 km2. Major sources of irrigation water are canals, tanks, tube wells etc. Besides that, groundwater can be used for agricultural purposes.

2.2 Data Collection

2.2.1 Climatic and Meteorological Data

The climatic and meteorological data consists of monthly mean maximum and minimum temperature, sunshine hours, radiation, wind speed and relative humidity. These data are taken from CLIMWAT 2.0 for 2005 and are taken as input parameters to determine ETo by Penman–Monteith method according to FAO guidelines.

2.2.2 Crop Data

The value of crop coefficient (Kc) was taken from FAO -Irrigation and Drainage paper 56 [4]. Planting and harvesting date for sugarcane is 15/09/2005 and 14/09/2006 respectively and the type of soil was medium (loam) soil.

2.3 Theory and Methods

The reference crop evapotranspiration is evaluated as in Eq. (1) (Reddy 2020).

$$ {\text{ETo}} = \, [0.{4}0{8}\,\Delta \, \left( {{\text{Rn }}{-}{\text{G}}} \right) \, + \, \gamma \, \left( {{9}00/{\text{T}} + {273}} \right) \, ({\text{e}}_{{\text{s}}}-{\text{e}}_{{\text{a}}} ){\text{ u}}_{{2}} \left] {/ \, } \right[\Delta \, + \, \gamma \, ({1} + 0.{\text{34u}}_{{2}} )] $$
(1)

where, ∆ = slope of the curve between saturation vapor pressure and temperature in [KPa/oC]; Rn = net heat radiation in [MJ/m2/day]; G = soil heat flux in [MJ/m2/day]; γ = psychometric constant in [KPa/°C]; (es−ea) = saturation vapor pressure deficit in [KPa]; T = mean air temperature in [°C], u2 = wind speed at a height of 2 m from the ground surface in [m/s].

The crop evapotranspiration (ETc) is estimated by multiplying Kc with reference crop evapotranspiration (ETo) given by Eq. (2) [4].

$$ {\text{ETc }} = {\text{ Kc }} \times {\text{ ETo}} $$
(2)

where, ETc is crop evapotranspiration under standard condition obtained from single crop coefficient approach in [mm]; Kc is crop coefficient.

Effective rainfall is the amount of water, which is directly consumed by any plant and it is calculated after accounting for all the losses i.e. initial abstraction, infiltration loss, percolation loss, evaporation loss etc. [9]. In this present study, effective rainfall was determined based on the fixed percentage method available in CROPWAT 8.0 as in Eq. (3).

$$ {\text{ER }} = {\text{ a}}\cdot{\text{ P}}_{{{\text{total}}}} $$
(3)

where, ER is effective rainfall in [mm]; a is fraction of total rainfall getting converted into ER (user defined parameter); Ptotal is total amount of rainfall in [mm].

3 Results and Discussions

3.1 Reference Crop Evapotranspiration and Effective Rainfall

Reference crop evapotranspiration had been evaluated based on the available climatic parameters for the year 2005 for Kurnool district. The monthly climatic and meteorological data, daily ETo is represented in Table 1 [3].

Table 1 Kurnool district: weather station data and reference crop evapotranspiration in 2005
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From Table 1, it is clear that reference crop evapotranspiration was increasing during January to June and decreasing during July to December. The highest ETo was observed in May i.e. 7.94 mm/day whereas it achieved its minimum value in December i.e. 3.4 mm/day. The differences in ETo values show the variation in the climatic parameters of this study area.

Low relative humidity, high monthly mean temperature, and high wind speed reflect increasing trend of evapotranspiration mainly in summer season [10].

The monthly effective rainfall was determined by the fixed percentage method as described in Eq. (3) by considering the value of ‘a’ as 0.8 and the results are shown in Fig. 1. In most of the cases, the effective rainfall is considered to be 80% of total rainfall for a rainfall value of less than 100 mm in a month [11]. In this present study, monthly rainfall value was below 100 mm except for the month of July, August and September in 2005 (Fig. 2) for Kurnool district. So, the value of the fraction (a) was taken as 0.8 as insufficient information is available from local conditions for this study area.

Fig. 1
A double bar graph plots rainfall in millimeters versus months. The bars are for monthly rainfall and effective rainfall. The highest bars are for the month of September with the bar for rainfall higher than effective rainfall.

Variation of monthly rainfall and effective rainfall of Kurnool district in 2005

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Fig. 2
A graph plots soil water retention in millimeters versus days after planting. The data is for R A M, T A M, and depletion. R A M is a constant line at 275 millimeters and T A M is a constant line at 440 millimeters. Depletion is the highest at 122 days. Values are approximated.

Soil water retention—growing length curve for sugarcane

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From Fig. 2, it can be observed that the distribution of effective rainfall shows almost uniform variation from January to March and then increasing from April to September and finally decreasing from September to December. Also, the rainfall distribution pattern for 2005 was irregular. Maximum effective rainfall depth was 102.4 mm in the month of September and there was negligible quantity of rainfall in the month of January, March and December. Almost 68.5% of effective rainfall was occurring in the monsoon season and around 20.6% was occurring during post-monsoon season.

3.2 Crop Water Requirement (CWR) and Irrigation Scheduling

Irrigation water for sugarcane was determined by considering available climatic, soil and crop parameters. Crops have different water needs based on the site for agriculture, soil texture, method of agriculture, climatic condition of that place, and effective rainfall and it is not uniformly distributed throughout the entire growing cycle of crops [12]. Irrigation schedule provides important information about the timing of irrigation, controlling the application and discharge of water in an effective and systematic way and also it improves the well-developed irrigation management in the field [3].

Tables 2 and 3 show the crop water requirement and irrigation schedule for sugarcane respectively.

Table 2 CWR for sugarcane
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Table 3 Irrigation schedule for sugarcane
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For sugarcane, effective rainfall only satisfied 23.6% of its crop water demand. So, a significant water requirement would be satisfied by irrigation. Being a perennial crop, it needs water throughout its entire growing period for its growth. Around 20.6 and 10.8% of effective rainfall satisfied the crop water demand for sugarcane during post-monsoon season of 2005 and pre-monsoon season of 2006 respectively. So, sugarcane can utilize a significant amount of water during monsoon period of 2006. A significant amount of water from irrigation was needed from January to June i.e. almost 1243.7 mm which constitutes 60% of crop water requirement. Kc value is more at the time of transplantation and it was significantly falling from its initial stage to crop development stage up to 2nd week of October. After that it would increase almost linearly up to the end of its mid-season stage and then was falling down from the end of mid-season to the end of late season stage of crop development at a lower slope. Initially, the crop requires less amount of water with a large irrigation interval as it is few centimeters high. So, a very small canopy covers the ground and Kc is low. Whereas, a considerable evaporation can take place from soil resulting high value of soil evaporation coefficient (Ke) at this phase [13]. But during the crop development and mid-season stage, almost maximum canopy (70–80%) covers the ground resulting in high leaf area index [4] and flowering stage starts at this phase. So, productivity will be high and significant transpiration takes place from the plant stomata [4].

Table 3 shows the irrigation schedule for sugarcane for the condition refill soil at field capacity and irrigation at critical depletion, available in CROPWAT 8.0. The first watering was done 124 days after its transplantation. The irrigation interval was fixed at 49 days, 38 days, 35 days and 43 days from the consecutive watering. Sugarcane has faced no significant moisture deficit at the root zone as Ks was unity. So, adjusted evapotranspiration almost meets crop evapotranspiration. Initially, the flow rate was low and it was significantly increased by 152.6% during its mid-season stage. Higher flow rate through irrigation can be observed during its mid-season stage (April–May) as effective rainfall was low to meet the water needs. NIR was obtained as 1423.2 mm and GIR was 2033.1 mm. In this present study, GIR is obtained by considering an application efficiency of 70%.

Net irrigation requirement (NIR) is the amount of water that can be needed for a plant to attain the water content of existing soil up to field capacity to enhance the growth of plant [3]. It can be calculated by considering an extra amount of water required for leaching with crop evapotranspiration (ETc). Gross irrigation requirement (GIR) is determined from NIR by adopting suitable irrigation efficiency based on the type of irrigation. Some portion of water is used by evaporation, surface runoff, interception loss as well as percolation loss. So, moisture deficit can be seen at the root zone resulting in depletion of total available water. Different irrigation water requirement is given in Eqs. (4), (5) and (6)

$$ {\text{ETc }} = {\text{ Cu }}-{\text{Eff}}.{\text{rain}} $$
(4)
$$ {\text{NIR }} = {\text{ ETc }}+{\text{ leaching requirement}} $$
(5)
$$ {\text{GIR }} = {\text{ NIR}}/\eta {\text{a}} $$
(6)

where, ETc is daily crop evapotranspiration [mm/day]; Cu is daily consumptive use of water [mm/day]; ηa is the application efficiency.

Figure 2 shows the soil water retention—growing length curve for sugarcane. In the figure, TAM is the total available moisture [mm]; RAW is the readily available moisture [mm]. RAW is a fraction of TAW which a plant takes from the soil moisture zone without any occurrence of water stress. TAM and RAM were uniform throughout the entire growing cycle of sugarcane resulting in the constant crop height and RAW was almost 64% of TAW. Initially, the moisture depletion was observed after 65 days from its transplantation. Therefore, it did not affect the crop growth, but at the time of harvesting it was low and the variation of moisture depletion showed a marginal difference throughout the initial to mid-season stage.

3.3 Crop Characteristics

Some major conservative and non-conservative crop characteristics were studied under limited set conditions in AQUACROP 6.1 model for sugarcane. Crop parameters are used to evaluate rooting depth, growth of canopy, biomass and crop yield [14]. Conservative crop parameters include base temperature, canopy development with crop period, biomass and yield. Whereas, non-conservative crop parameters may consist of management dependent parameters, crop phenology and soil characteristics [5]. AQUACROP model shows variation of canopy growth profile with transpiration rate by considering canopy development, leaf area expansion, senescence and harvest index [5]. In AQUACROP, yield is obtained by multiplying the harvest index with the biomass and biomass can be determined from Eq. (7).

$$ {\text{B }} = {\text{ WP*}} \, \times \sum {\frac{\text{Tr}}{{\text{ETo}}}} $$
(7)

where, B is the daily biomass [ton/ha]; WP* is normalized water productivity at an atmospheric CO2 concentration of 369.41 ppm [g/m2]; Tr is the daily crop transpiration [mm] [5].

Table 4 shows the crop characteristics for sugarcane. Initially, due to high initial canopy cover, LAI was high resulting in more transplantation from the plant stomata. Maximum canopy cover was achieved at the crop development stage and the crop senescence occurred at 266 days after the formation of maximum canopy cover. The rate of root zone development was faster for sugarcane. Hence, maximum crop height could be achieved within the minimum possible time after transplantation. Being a C4 type crop, the tangent of the line between biomass and ∑\(\frac{\mathrm{Tr}}{\mathrm{ETo}}\) is high as compared to C3 type of crop due to a less efficient carbon assimilation process resulting in high WP* [6]. No water stress was seen during canopy expansion, stomatal closure, and early senescence resulting in high production of crops, thus, actual biomass production tends to be the same as the potential biomass. The growth of canopy cover shows a non-linear relationship with the growing period for sugarcane as given by Eq. (8)

$$ {\text{GCC }} = -0.00{\text{34B}}^{{2}} + { 1}.{\text{4412B }}-{ 18}.{672} $$
(8)
Table 4 Crop characteristics for sugarcane
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where, GCC is the growth of canopy cover [%] and B is the crop growing period [days].

Growth of canopy cover (GCC) basically shows the canopy development at different stages of crop growth starting from emergence to its full maturity. Prior to harvesting, canopy senescence occurs and maximum canopy cover can be attained during flowering stage.

The root zone development for sugarcane shows a linear relationship with the growing period according to Eq. (9) (Fig. 3)

$$ {\text{Zr }} = \, 0.0{453}\cdot{\text{B}} + 0.{2798} $$
(9)
Fig. 3
A line graph plots root zone in meters versus growing length in days. The trend is inclining. The root zone is equal to 0.0453 x + 0.2798 with r square equal to 0.9835.

Variation of Root zone development with growing length for sugarcane

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where, Zr is the root zone depth [m].

4 Conclusion

The evaluation of crop water requirement, irrigation schedule and crop characteristics were studied in this present study for sugarcane. All the simulations were performed in CROPWAT 8.0 and AQUACROP 6.1 model. The major outcomes of this research are being discussed below:

  1. (1)

    Average daily reference crop evapotranspiration (ETo) was 5.37 mm/day and it shows an increasing trend during dry season and vice-versa.

  2. (2)

    A significant amount of crop water demand was satisfied by irrigation for sugarcane being a perennial crop.

  3. (3)

    CWR for sugarcane was estimated as 2090 mm. It will require more amount of water for its maturity and consequently, it will have more ETc.

  4. (4)

    Root zone development shows a linear variation with the growing period for sugarcane whereas variation of GCC was non-linear with crop growing period.