Estimation of water requirement of Banana crop under drip irrigation with and without plastic mulch using dual crop coefficient approach

Field experiments were conducted on the lateritic sandy loam soil and sub-humid climate of Kharagpur, West Bengal, India during 2014-2018 for two consecutive crop seasons to determine the optimum water requirement of the Banana crop under drip irrigation with and without plastic mulch. Reference evapotranspiration for the Banana crop was estimated using FAO-56 Penman-Monteith approach. The dual crop coefficient approach was used to improve the estimates of the crop coefficient (Kc) values for the irrigation treatments with black plastic mulch and without mulch. Plants treated with black plastic mulch consume about 12% less water for the main crop and about 22% less water for ratoon crop compared to non-mulch plants. The effect of black plastic mulch was studied to determine the soil moisture distribution in different soil depths (0-20, 20-40, 40-60, and 60-90 cm), and also loss of water through deep drainage. Net irrigation requirement under black plastic mulch was found to be lesser than without mulch. The deep percolation was found 50.4 % more under black plastic mulch in comparison to non-mulch treatment.


Introduction
Banana (Musa paradisica L.) is one of the oldest fruit known to humanity, which is a rich source of carbohydrates, vitamins, and minerals essential for the human diet. In 2014, worldwide, banana production was about 16.5 million tones [1]. India ranks first in the banana producing countries followed by China, Philippines, Ecuador, Brazil, and Indonesia. Drip irrigation has the potential of precise application of water both in amount and uniform application throughout a field [2]. Plastic mulch influences the microclimate near plants, temperature, net radiation, relative humidity [3], and affects crop water requirement and water productivity of plants [4]. The improved estimates of crop coefficients are needed for accurate irrigation scheduling for high valued crops daily. The FAO-56 dual crop coefficient method separately estimates transpiration and evaporation, which can be used for irrigating crops [5].
Researchers have investigated dual crop coefficients under drip irrigation with mulching [6,7]. However, research on dual crop coefficient for banana under mulched drip irrigation has not been reported in the literature. Hence, the present study aims to i) estimate banana crop water requirement using the dual crop coefficient approach and ii) to evaluate water balance parameters under black plastic mulch and non-mulch conditions.

Materials and methods
Field experiments were conducted for two crop seasons (2014-2018) at the Experimental Farm of Precision Farming Development Centre, Indian Institute of Technology, Kharagpur, India to estimate crop water requirement of banana under drip irrigation system and black plastic mulch. The experimental site is at 22°18.5' N latitude, 87°19' E longitude and at an altitude of 48 m above mean sea level. Annual rainfall varies from 1200 to 1500 mm and about 80% of this rainfall occurs during June to October. The monthly average of daily minimum air temperature is 6°C in January, whereas the monthly average of daily maximum air temperature is 43.5°C in May.
The daily meteorological data recorded from 2014 to 2018 were utilized to compute reference evapotranspiration (ET 0 ) using the modified Penman-Monteith method, as suggested by [8]. The dual crop coefficient is a summation of a basal crop coefficient (K cb ) and soil evaporation coefficient (K e ). This approach estimates the plant and soil components of the crop coefficient separately and quantifies both the components independently and allows to make a comparison between them [9].
The basal crop coefficient (K cb ) is the ratio of crop evapotranspiration and the reference evapotranspiration (ET c /ET o ) while the soil surface is dry, but transpiration occurs at a potential rate in a plant. The soil evaporation coefficient (K e ) is the evaporation component of ET c . Where the top soil is wet, following rain or irrigation, K e is maximal. Where the soil surface is dry, K e is small and even zero when no water remains near the soil surface for evaporation.
The net irrigation requirement can be computed as follows Where I w -Net volume of irrigation (L); Ap -Effective area per plant considering wetting fraction (m 2 ); R e -Effective rainfall (mm). The field experiment was carried out for two successive crop seasons to estimate the daily crop water requirement under drip irrigation and plastic mulch. Tissue culture banana plants were transplanted in the field at a spacing of 2 m × 2 m in a plot size of 240 m 2 with drip system and a bed covered with 50-micron black plastic mulch. Sixty plants each under black plastic mulch (BPM) and without mulch (NM) were selected for the study.
The water balance study carried out using the following soil water balance equation where P -rainfall,(mm); I-irrigation water supplied with drip system,(mm); ET c -crop evapotranspiration, (mm); ∆S -change in soil water storage in the soil profile, (mm); DP -water lost due to deep percolation; R-surface water runoff, (mm). Soil moisture content was measured at different soil depths (0-20, 20-40, 40-60, 60-90 cm) using FDR (frequency domain reflectometry) sensor for the period of the second crop season. During the experimental period, the deep drainage loss was computed using unsaturated hydraulic conductivity as a function of the prevailing moisture content and time period. The unsaturated hydraulic conductivity was estimated using van Genuchten equation Where Se is the relative saturation given by Where Ks = Saturated hydraulic conductivity (mm day -1 ), θ = Volumetric soil water content (mm 3 mm -3 ), θs = Saturated volumetric water content (mm 3 mm -3 ), θr = Residual volumetric water content (mm 3 mm -3 ), l = Pore connectivity/tortuosity parameter and m = van Genuchten parameter. These were obtained from the soil moisture characteristic curve, field measurements, and hydraulic parameter optimization using software RETC (RETension Curve). Parameters values obtained by RTEC software were θr = 0.0447, θs = 0.3581, Ks = 0.34 and l= 0.5. The deep percolation loss (DP) was estimated by equation (6).
Where, q = Mean volumetric flux density (mm day -1 ); Δt = Time period (days). For periods without rainfall, runoff value is zero and during wet periods runoff was estimated by curve number (CN) method [5].

Determination of crop coefficient
The daily meteorological data recorded during two crop seasons of 2014 -2016 and 2016-2018 were used to estimate ET 0 (figure 1). It can be seen from figure 1 that average ET 0 values from February to September are comparatively greater than other months. The ET 0 value gradually increases with the increase in sunshine hours and the intensity of radiation. The average peak value of the ET 0 was 5.1 mm and 5.36 mm occurred in May during 2014-16 and 2016-2018, respectively. From June onward average daily ET 0 values were observed to be reduced due to the incidence of rainfall, low solar radiation, and high humidity, especially during monsoon months (June to September). Further due to the lowering of temperature from October onward till March, daily ET 0 values gradually reduced and reached to lowest in December. According to FAO-56 recommendation, the values of basal crop coefficient (K cb ) for initial, development, and end of the growing season (K cbini , K cbmid, and K cbend ), for the banana crop were 0.15, 1.05 and 0.9, respectively. Initial K cb value was comparatively lower due to lesser leaf area and value increases with an increase in leaf area. According to Shrestha and Shukla [7], the K cb increased considerably due to increased transpiration as crop went through rapid plant growth phase to reach effective full cover.    [10] and Mata et al [11].
Based on the results reported in table 1, ET c values of BPM treated plants were obtained to be lower than that of NM plants at the initial stage of main and ratoon crops due to more soil surface  5 available for evaporation which was reduced by the BPM. The rapid growth of plants can be observed in mulch treated treatments during the mid-season period of both main and ratoon crop leading to the higher ET c in comparison to non-mulch condition. Overall, total ET c for the non-mulch treatment was greater than the mulched treatment during the whole period of main and ratoon crop. Many studies have reported that the ET c is strongly correlated to the available energy supplying for the dense crop canopy [12,13]. In this study, plastic film decreased available energy supplying the latent heat flux, and thus reduced ET c . Earlier studies also reported the reduction in soil evaporation by 40-70% in Squash [14], 46-60% in potato [15], 40-70% in cucumber [16] and 40-70% in okra [17] due to the application of plastic mulch. Cumulative transpiration values show contrast trend compare to total soil evaporation. There is an increase in total transpiration amount by 14.1% and 11% in main and ratoon banana crops respectively during the first crop cycle. The increase in transpiration amount up to 35% in potato [15], by 15% -30% in cucumber [16] and 10% -30% on okra [17] have been reported due to the application of BPM along with drip irrigation.
Total crop evapotranspiration (ET c ) is the product of daily ET 0 and daily K c values of main and ratoon crop for two consecutive crop cycles. It is also the summation of total evaporation and total transpiration during the crop cycle. There was no significant difference found between ET c of BPM treatments, ET c of NM irrigation treatments in first crop cycle (2014 -2016). Application of black plastic mulch reduces the soil water evaporation significantly, but BPM treated plants experiences rapid growth in comparison to that NM treated plants, which lead to an increase in transpiration amount. Because of the reduced evaporation and increased transpiration, no significant variation was found in total ET c of BPM treatments in comparison to NM treatments. These results are also in agreement with those of Jenni et al. [18] and Orzolek [19].
The deep percolation loss varied considerably due to irrigation, rainfall, and plastic mulch application during the crop cycle. Table 2 shows deep percolated water below the root depth in BPM treatment was 23.1% more for main crop and 24.8% more for ratoon crop in comparison to corresponding NM treatments in the first crop season. The increment in deep percolation loss at root depth of banana crop was 50.4% for the main crop, and 30.4% for ratoon crop in BPM treated irrigation treatments in comparison to NM treatments in the second crop season. The surface runoff was estimated for both main and ratoon crops during the first and second crop season. No significant difference in surface runoff magnitude was obtained during the irrigation period in BPM and NM treatments; this may be due to a water supply through the drip system as it does not generate surface runoff. However, a runoff was obtained during the rainy season both in BPM and NM treatments.

Conclusion
This research study established a dual crop coefficient for the banana crop. The maximum evapotranspiration of banana crop is obtained 5.67 mm in May, and the minimum value is 1.73 mm in October (which is the initial growth stage). The crop evapotranspiration estimated can be used to determine the amount of irrigation water to be supplied to the banana crop in different months. Application of plastic mulch reduces crop evapotranspiration by 14% and increases the deep percolation by 50.4% in comparison to non-plastic mulch.