Determination of actual evapotranspiration for summer clusterbean (Cyamopsis tetragonoloba) using mini-lysimeters in hot arid zone of India

Experimental site and crop growing environment
 
The field experiment was conducted at Research Farm of ICAR- Central Arid Zone Research Institute, Jodhpur (26.3°N and 73.02°E; 224 m amsl) during the summer seasons of 2015 to 2017. Soils of the experimental plots were originated from rhyolite and subsequently modified through alluvial and aeolian processes. Taxonomically, soils may be defined as coarse loamy mixed hyperthermic of camborthids. Soil organic carbon is very low (1.6 g kg^-1). Soil physical properties of the experimental area are shown in (Table 1).The climate of the region is arid characterized by high diurnal and seasonal temperature variations and annual and inter-annual irregular rainfall with long dry seasons associated with strong winds. During the crop growing season in 2015 four rain events accounted for 38.7 mm rainfall while no rain event occurred in 2016 and in 2017 rainfall occurred 86.5 mm with 5 rainy days (Fig 1). Relative humidity (RH) varied from 15.5-84, 11-51 and 13-67% in 2015, 2016 and 2017, respectively. Low relative humidity occurred in 2016 due to not a single rainfall event occurrence. Minimum temperature varied from 15-31.7, 15.2-33.2 and 10.5-30.9°C and maximum temperature varied from 27-44.7, 30.2-49.4 and 27.7-44.5°C in 2015, 2016 and 2017, respectively (Fig 1). 
 

 

 
Crop experiment details
 
Clusterbean cv. RGC 936 was planted after pre-sowing irrigation (60 mm) on 11^th March during 2015, 2^nd March in 2016 and 4^th March in 2017 using seed rate of 10-12 kg ha^-1. Plant spacing was 50 cm row to row and plant to plant 10 cm. Fertilizer (Di-ammonium phosphate-D.A.P) was applied at the rate of 20 kg N and 40 kg P₂O₅ ha^-1.The other cultural practices followed were kept uniform as per recommendations for all the treatments.The crop was harvested on 1^st June, 25^th May and 25^th May in 2015, 2016 and 2017, respectively. Treatments consisted of 4 irrigation levels, viz. irrigation at 100% (I_100%), 80% (I_80%), 60% (I_60%) and 40% (I_40%). The irrigation was given to the crops, when cumulative potential evaporation value reached 50±10 mm because the high rate of open pan evaporation (reached upto 20-25 mm/day) in summer season in hot region.
 
Soil moisture under different treatments was recorded periodically through FDR soil moisture meter of Micro Gopher.
 
Design and development of mini-lysimeter
 
Four mini-lysimeters (ML) were designed and fabricated for quantifying actual measurement of water balance components. Electronic weighing mini-lysimeter comprised three components (i) mini-lysimeter chamber/ pit, (ii) electronic weighing machine based on single load cell and (iii) mini-lysimeter tank. The tanks were designed to quantify actual water balance components from the tanks after irrigation and rainfall event. The tank dimension was 0.50 m × 0.50 m × 0.55 m, including 0.05 m of drainage chamber height, with 10 cm height supporting legs.
 
Electronic weighing machine of the mini-lysimeter was based on load cell (Make-ADI Company India; regular 410), having the weighing capacity up to 750 kg with a resolution of 50 g. It was fabricated according to our requirement within platform size 0.50 m × 0.50 m × 0.27 m. The platform was made of 4 mm mild steel (MS) iron and MS angle for supporting legs. The single load cell was bolted in an upright orientation in the centre of platform. The load cell gives output in millivolts with a connected EG Tech display indicator and is converted in kg, which is displayed by the system indicator. All the components were assembled to operate electronic weighing mini-lysimeter based on the single load cell. Soil up to 50 cm profile from the ground level from the experimental plot was excavated to fill the ML. Tank filled-in soil layer was alternatively saturated and drained until the bulk density inside the ML was close to the field soil condition. The mini-lysimeter was calibrated in field with conversion factor of 1 kg = 4 mm with resolution of 0.2 mm (Meena et al., 2015).
 
Established mini-sprinkler irrigation system
 
Mini sprinkler irrigation system setup was made in the experimental field, which  consisted of pump (1 HP), main pipe (HDPE pipe 63 mm diameter; 2.5 kg/cm² pressure), screen filter (25 m³/hr), low linear density polyethylene (LLDPE) plain lateral (32 mm; 2.5Kg/cm² ), risers, sprinkler head and pressure gauge. Twin nozzle mini-sprinklers of model Monsoon S-10 with nozzle size 2.5 × 1.5 mm were used in the field to irrigate the area. The mini-sprinklers were used in a part circle. The sprinkler fixed at the middle of the individual plot was fixed at 180°, while sprinklers at the corner of the plot were fixed at 90° angle. For each irrigation level six sprinklers were used, out of that, 2 sprinklers were fixed at 180° and four at 90°. The net area under one irrigation level was kept as 21 m × 11 m. The spacing of sprinklers along the lateral was kept as 10.5 m and spacing of laterals along the main line was kept at 10 m. One meter gap was left between the rows of two irrigation levels plots.The experiment was conducted at operating pressures of 2.0 kg/cm² at the nozzle. Discharge of a single nozzle was found to be 480 lph. Precipitation rate of the sprinkler system was calculated to be 4.6 mm/hr with the use of following equation.  
Where,
Precipitation rate (mm/hr); Discharge (m³/hr) and wetted area (m²).
 
Based on the flow discharge of sprinkler nozzle, the flow rate of 6 sprinkles for the individual plot was calculated as 2880 lph. Total water to be applied on the individual plot was calculated to be 10500 litres at 50 mm depth of irrigation, hence, time of operation of sprinkler system for irrigation levels of I₁₀₀, I₈₀, I₆₀ and I_40% was calculated to be 3.6, 2.9, 2.2 and 1.5 hrs, respectively. Assessment of water productivity was done as defined by Araya et al. (2011) and Payero et al. (2009). Where,
Ya is the actual yield achieved (kg ha^-1) and ET is water used in evapotranspiration (mm).

Plant water potential (PWP) and Relative leaf water content (RLWC)
Sampling for assessment of plant water status was done in triplicate prior to subsequent irrigation. Observations on plant water status were done at flowering and pod maturation stage as they have been reported to be critical for yield (Garg and Burman, 2002).
 
Plants were uprooted and water potential was measured using a pressure chamber (PMS Instrument Co., USA). Relative leaf water content (RLWC) was also determined at these two stages from fully expanded leaves (third from top) following the procedure of Barrs (1968), wherein 200 mg of fresh samples of leaf disc (FW) were floated on equal amount of  distilled water for 3 h to obtain turgid weight (TW). After oven drying for 24h at 80°C the dry weight (DW) was recorded. Thereafter LRWC was calculated as –

Daily actual crop evapotranspiration
 
Daily crop ET was measured with installed mini-lysimeter under irrigation levels. The variation of daily average ET_C under irrigation levels was 8.0, 6.9, 5.3 and 3.8 mm/day in 2015, respectively. In summer 2016 was 8.5, 6.8, 5.6 and 4.0 mm/day while in 2017, it was 8.0. 6.3, 6.1 and 5.0 mm/day, respectively. It was observed over the three summer seasons that highest average ET_C (mm/day) under irrigation levels in 2016 as compared to 2015 and 2017. Also observed maximum ET_C reached 20 (mm/ day) was in 2016 followed by 2017 (19.6 mm/day) and 2015 (19.2 mm/day) (Fig 2). The highest ET_C in 2016 occurred due to low relative humidity and high air maximum temperature and these were influenced due to no rain events particularly in the summer season (Fig 1). Crop growth and yield was also higher in summer 2016 because it is a photosensitive and indeterminate crop.
 

 
Clusterbean yield
 
Perusal of Table 2 shows that water consumed for actual evapotranspiration (ET) was maximum with treatment T1, where 100% water was applied to the plant on volumetric basis, and the lowest was for T₄, where 40% of water was applied in all the years. Mean ET value across the years was maximum (686 mm) for T₁ and the lowest (340 mm) for the treatment T₄. As can be observed from the table, grain yields decreased as the amount of irrigation water applied decreased. The highest yield, averaging 2149 kg ha^-1, was measured in the fully irrigated treatment, T₁, while water deficit treatment T₄ produced the lowest yield of 680 kg ha^-1. Clusterbean yield for treatment T₂ (I80%) was very close to that of T₁, but yield in other treatments T₃ (I_60%) and T₄ (I_40%) were very less in comparison to T₁.
 

 
Water productivity
 
The seasonal ET varied in all the years for the different irrigation treatments. Crop water use for treatments T₁, T₂, T₃ and T₄ was 673, 574, 442 and 313 mm, respectively, in 2015. In 2016 season, the respective values for these treatments were 723, 577, 425 and 299 mm, while for 2017 season these values were measured to be 663, 512, 494 and 407 mm. The mean value of crop water use (ET) for the treatments was 686, 554, 454 and 340 mm. The marked differences observed among the treatments, can be attributed to the large difference in irrigation water applied. The mean water productivity ranged from 0.21 (T₄) to 0.35 kg m^-3 (T₂). In a study done by Kumar et al., (2016) water productivity of kharif clusterbean in hot arid region was found to be 0.38 kg m^-3, which is similar to our results in this study.
 
The values estimated for water productivity have some very important implications. Under a limited water supply situation where the goal may be to achieve the highest possible water productivity, utilizing a water application depth of 40 mm (T₂) at each irrigation event offers opportunities for water savings. In other words, utilizing this water application depth offers water savings of 19.24% (Table 2) compared to the fully irrigated treatment with only 10.46 yield reductions. In other words, if yield is to be maximized, crop ET would need to be about 686 mm (3 year average), or 23.8% greater than the water use at maximum water productivity (Table 2).
 
Soil moisture
 
Yearly variation in the pattern of soil moisture across different treatments (T₁ through T₄) was observed at both the critical stages (flowering and pod maturation stage) of crop growth (Table 3). In 2015, soil moisture progressively decreased with increasing deficit of irrigation at flowering stage. Similar trend was broadly observed at pod maturation stage too. Slightly high value in T₂ and T₃ could be due to less plant population resulting in more residual soil moisture. In T₄, however, deficit irrigation was so severe that soil moisture declined further probably in spite of less plant population. More soil moisture in T₁ and less in T₄ were also observed in 2016 at both the growth stages. Intermediate levels of irrigation (T₂ and T₃) reflected inconsistent trend probably because of variation in plant population in these treatments. In 2017, the interplay between rain fall events and plant population strongly influenced the soil moisture as at pod maturation stage probably due to last 2-3 rainfall events prior to harvest (Fig 1).
 

 
Plant water potential and relative leaf water content
 
As the intensity of deficit irrigation increased from 0.0% to 60% (i.e. at 100 and 40% CPE irrigation level, respectively) the plant generally experienced increase in water deficit which was maximum at 40% level (Table 4). However, variability with respect to plant water deficit on different sampling days could be explained due to variation in rainfall event, plant population in different treatments and consequent soil moisture level.  High plant water potential at pod maturation compared to that at flowering stage in 2017 is a clear-cut example wherein rainfall events and the associated increase in relative humidity and mean temperature (Fig 1) resulted in less plant water deficit.
 

 
With progressive decrease in irrigation level the clusterbean plants experienced more plant water deficit. On an average of plant water potential dropped from -17.8 to - 20.1 bars (Table 4). Further, clusterbean plants also reflected their potential to adapt as evident from themaintenance of comparable relative water content in its leaves even at lower levels of irrigation (Table 5). But maintenance of comparable level of plant water status could not check decline in yield. It is pertinent to add here that yield reduction was less at 80% level probably because deficit irrigation reduced leaf area (consequently checking transpiration) besides improvement in root growth (data not presented). It is well known that a specific level of soil moisture deficit will not necessarily be accompanied by same degree of water stress and this in turn does not influence all aspects of growth equally (Turner and Begg, 1981). Literature reveals that shoot growth invariably decreases, whereas root growth increases till an acceptable level of water deficit (Krida, 2002; Nangare et al., 2016) followed by yield reduction.
 

Soil water availability is a major limiting factor in arid agricultural production systems. Knowledge of crop response to water supply, full and limited, in localized environments can aid in the development of effective irrigation strategies for improving water management and crop productivity. Based on a mini-lysimetric approach for direct measurement of crop water use or actual evapotranspiration, a water application depth of 40 mm (I_80%) showed highest water productivity, while a 30 (I_60%) and 20 (I_40%) mm water application depth was insufficient to maintain a wet soil profile, resulting in lower crop water productivity value with 0.25 and 0.21 kg m^-3. The results also indicate that to achieve maximum water productivity, crop ET would need to be at least 554 mm. High water productivity is attainable without significant yield reduction at water application depth of 40 mm, offering opportunities for improving farm level water use and saving of water resources.From the present study, it may be inferred that that the crop can save 19.2% (132 mm) of water with a compromise in yield reduction by 10.4% (225 kg). Therefore, clusterbean crop can be grown profitably during summer season with 80% of irrigation level in the arid zone of India.