Rainwater harvesting and its efficient utilization in mandarin (Citrus reticulata Blanco) orchards under a dry sub-humid tropical climate

Water is an important natural resource and a pre-requisite for the development and subsistence of mankind. The higher water demand for burgling population, exponential industrial growth, and superior life style of people put tremendous pressure on water resources. The present share of water for agriculture is going to be reduced in the coming decades in different parts of the world. On the other hand, to feed the increased population, higher production of food grains, vegetables, fruits, and other allied agricultural produces is indispensable. In this scenario, producing more with less water becomes a need.

Citrus is an important fruit crop that is grown in around 140 countries of the world. The major citrus-grown areas are confined to tropical and subtropical regions, where water scarcity is a common problem. The citrus plant requires around 1,000–1,200 mm of water annually for its growth. Water shortage in its critical growth stages (flowering, fruit setting, and fruit growth) affects the yield and quality of citrus fruits. Thus, optimal water supply with efficient use of water is the need of the hour for sustainable production of citrus in water-scarce regions.

India is the third largest citrus producing country after China and Brazil in the world. However, the productivity of citrus (12.75 t ha⁻¹) in India is very low in comparison to that in Turkey (31.65 t ha⁻¹), Brazil (25.78 t ha⁻¹) and USA (25.11 t ha⁻¹). The central part of India is a major hub for citrus production in the country. Nagpur mandarin (Citrus reticulate Blanco), a world-famous loose skin citrus cultivar, which is used as a table fruit, is primarily cultivated in around 0.2 million hectares of central India. Due to climate suitability and higher financial return compared with other crops, the area coverage under citrus has increased substantially in the region during the last few decades (Panigrahi et al. 2012). However, water scarcity in the summer months causes the drying of citrus orchards and results in a substantial financial loss to the growers. Moreover, an overdraft of groundwater has resulted in a sharp decline in water levels in the aquifers of those regions (Panigrahi & Srivastava 2011). Earlier, Panigrahi et al. (2009, 2012) advocated for the adoption of drip irrigation and fertigation, which could save 35% of water and 25% nutrients, compared with traditional surface irrigation and fertilization on the crop. Moreover, solar energy based drip irrigation is an option for energy saving and environmentally friendly crop production. Mulching (Panigrahi et al. 2008) and deficit irrigation (Panigrahi & Srivastava 2016; Panigrahi 2023) were also found to be effective soil moisture conservation and water saving techniques in citrus. Though the annual rainfall in the region is around 960 mm, the major portion (86%) of it is confined to monsoon months (July–October). Out of the total rainfall amount, more than 60% of it goes as runoff along with the substantial top fertile soil from the orchard (Panigrahi et al. 2017). There is a lot of scope to harvest and conserve the rainwater in situ and ex situ, and the reuse of harvested water during critical periods of the crop to sustain and enhance citrus production in the region.

Rainwater harvesting (RWH) is the technique of collecting and storing rainwater for subsequent reuse in households, agriculture, and other purposes in an area (Murty 1998). The RWH system consists of the following three elements: (i) catchment: an area from which the excess rainwater (runoff) of a land surface is generated for collection, (ii) transferral system: a system through which the excess rainwater generated is transferred or conveyed for storage, and (iii) storage system: a space or an area used for storing the runoff (Suresh 1997). In agriculture or crop production, basically, there are two methods of RWH: (i) in situ and (ii) ex situ. In the in situ method, rainwater is harvested where it falls, using a bund, trench, etc. in the crop field; whereas, in the ex situ method, rainfall-runoff generated from the catchment/crop area is conveyed through a conveyance system and harvested outside of the crop field, using tank, farm pond, check dam, etc. for further use (Singh et al. 2011). Sometimes, the use of both in situ and ex situ methods of RWH becomes beneficial.

Earlier, many studies have reported the effects of rainwater conservation and its effects on citrus orchards. Wang et al. (2010) reported that among different inter-crops viz., Bahia grass, peanut, and citron daylily, Bahia grass conserved the maximum runoff in citrus orchards. A study conducted to evaluate the effects of straw mulch in citrus orchards indicated that mulch could conserve the runoff without bringing any changes in the yield parameters of citrus trees (Liu et al. 2012). The performance evaluation of different conservation practices such as (i) bare land (BL), (2) citrus plantation without grass cover (WG), (3) citrus plantation with strip planting of Bermuda grass (SPB), and (4) citrus plantation with full coverage of Bermuda grass (FCB) indicated that FCB was the most effective by conserving 99% of runoff in citrus orchards (Mo et al. 2019). Duan et al. (2020) studied the effects of grass cover and cover crops on runoff in citrus plantations and reported that grass cover was more effective in conserving 47% higher runoff than cover crops. Wu et al. (2021) conducted an experiment to evaluate the effects of Bermuda grass strips in between tree rows, full coverage of Bermuda grass between tree rows, and crop rotation of radish and soybean with strip plantation between tree rows, and indicated that full coverage of Bermuda grass was the most effective in conserving runoff and improving soil microbes, compared with bare soil in citrus orchards in China. The results of different studies indicate that conservation measures perform differentially in relation to runoff conservation in various agro-climates. Limited studies are available on the impact of ex situ conservation measures (water harvesting tank, check dam, etc.) and integrated effects of in situ and ex situ measures on runoff conservation and its impact on productivity, water productivity, and economics of citrus production.

The prediction of overland flow has a vital role in the planning, design, and management of soil and water conservation treatments in any watershed, river basin, or catchment, where hydrological data are not available. At present, some simulation models are available to predict the overland flow in any hydrological unit. However, owing to the complexity and huge input data requirement for simulation, these models are seldom preferred by planners. Especially, in many developing countries where data scarcity is a common scenario, simple empirical models based on regression of the observed data serve as an alternative, due to their ease in predicting runoff (Mohanty et al. 2010). Such empirical models to predict the overland flow are not available for mandarin orchards of the study region. Keeping this in mind, the present study has been conducted to evaluate the performance of both in situ and ex situ rainwater harvesting measures and recycling of the harvested water in citrus orchards in a sub-humid dry tropical region of central India. The prediction of surface runoff from rainfall quantity in citrus orchards has also been done in the present study.

The performance of rainwater management strategies (RWMS) evaluated in the citrus orchard was as follows: (1) a continuous trench between plant rows (CTPR), (2) CTPR + rain water harvesting tank (RWHT) based surface irrigation (IS), and (3) CTPR + RWHT based solar-powered drip irrigation (ISD). The rain-fed treatment (RFT) in the crop was taken for comparison. All four treatments were laid out following a randomized block design (RBD) in the citrus orchard. The experimental orchard having 320 mandarin plants in 11,520 m² area (240 m × 48 m) was selected for each treatment. All plots were surrounded by bunds to restrict the runoff within the area. In the CTPR treatment, trenches of trapezoidal shape with 0.30 m depth, 0.60 m top width, and 0.15 m bottom width were made across the slope. To collect the runoff, RWHT with 3,675 m³ capacity (35 m × 35 m × 3 m) was excavated at the outlets of each plot.

Meteorological parameters including rainfall and evaporation were measured from the automatic weather observatory installed at around 200 m away from the experimental site. The volume of runoff was quantified using a multi-slot divisor at the outlet of each plot, and soil loss was worked out by determining the quantity of sediment per unit quantity of runoff water after each rainfall event. Water collected using the multi-slot divisor was pumped out to WHT after each rainfall. The water balance of the WHT was determined by quantifying the water harvested and lost through seepage/percolation and evaporation by installing a measuring staff (PVC pipe with 75 mm diameter and 4.0 m length) at the center of the tank. The moisture content within the crop root zone (up to 1.0 m depth) was quantified using a neutron moisture meter (Troxler, USA) and the average moisture content on a monthly basis was calculated. The quantification of soil moisture in crop root zones helps to understand the soil-water dynamics and response of plants to it under different water harvesting (CTPR, CTPR + RWHT) and irrigation methods (ISD and IS).

In recent years, energy has become a scarce resource due to its increased demand for industries, human use, agricultural operations, etc. Harnessing solar energy and its efficient use in pumping irrigation water is the need of the hour to sustain agricultural production in the coming years. The analysis of energy use, therefore, becomes necessary to determine and adopt energy use efficient techniques in any agricultural production system. The energy input (EI), energy output (EO), and energy use efficiency (ratio of EO to EI) under different treatments were estimated following the standard procedure (Singh et al. 1997; Namdari et al. 2011). The EI was worked out as the summation of the energy equivalent of various inputs required in the production system, whereas the EO was calculated as the energy equivalent of the output/yield under different treatments.

Financial analysis of crop production under any measure is important in farmer's perspective. Financial indicators such as gross income (GI), net income (NI), and benefit-cost ratio under different treatments in citrus orchards were determined following the standard procedure suggested by Panigrahi et al. (2013). The cost of production was worked out as the sum of investments in capital items and operational items along with the annual bank interest of 6% on the total investment. The capital cost of production was determined based on the investments done in installing solar-powered DI systems (INR 80,000 ha⁻¹) and the construction of WHT (INR 230,000 ha⁻¹) in the orchard. The annual capital cost of production was estimated considering the productive life of the drip irrigation system and WHT as 6 and 25 years, respectively. The annual operational cost of production was estimated as the sum of investments done in land preparation, trench making, fertilizers, insecticides, pesticides, weeding, irrigation, harvesting of fruits, etc. Under SI, the cost of manpower (INR 300 per person per day) involved in irrigating the orchard was considered under operational cost. The gross income was worked out based on the average wholesale price (INR 13,600 t⁻¹) of citrus fruits in the local market and yield under different treatments. The net income was calculated by subtracting the total cost of production from gross income. The benefit-cost ratio was determined as the ratio of gross income to the total cost of production. Economic water productivity was estimated as the ratio of income to water used under different treatments.

The relationships made between the quantity of rainfall and runoff under different treatments show that it was linear (R² = 0.82–0.91) under CTPR and CTPR + RWHT, and exponential (R² = 0.86) under RFT during 2004–2008 (Table 1). The exponential trend indicated that the runoff generated in RFT was higher than CTPR and CTPR + RWHT for any erosive rainfall amount in the orchard. A past study also highlighted the exponential relationship (R² = 0.84–0.88) between rainfall and runoff quantities in citrus plantations (Cerdà et al. 2009). Furthermore, the regressed equations developed between rainfall and runoff amounts reasonably predicted (R² = 0.84–0.94) the runoff quantities under different treatments during the years of 2009 and 2010 (Table 1). It indicates that these equations may be utilized to predict the overland flow, which may be helpful in designing and constructing water conservation measures in citrus groves in the present study region. The linear equations developed between runoff and rainfall under different conservation treatments indicated that the slope of the straight line was the highest in CTPR + RWHT + ISD (0.548), followed by CTPR + RWHT + IS (0.414). The highest slope indicated that the change in runoff quantity with respect to the rainfall amount was the highest in CTPR + RWHT + ISD and the lowest in CTPR.

The annual water use, total fruit yield, water productivity, suitable yield index, and energy use efficiency in citrus production under different treatments are given in Table 2. The highest water use (6,565 m³ ha⁻¹) was observed in CTPR + RWHT + ISD, due to rainwater harvesting and the reuse of harvested water in citrus plants in this treatment. Water used under CTPR + RWHT + IS was lower than CTPR + RWHT + ISD, due to the reduced water loss through deep percolation and evaporation under ISD compared with IS. The highest fruit yield (22.81 t ha⁻¹) was recorded in CTPR + RWHT + ISD, followed by CTPR + RWHT + IS (17.25 t ha⁻¹). The highest fruit yield under ISD was probably due to a higher photosynthesis rate and the partitioning of a higher portion of photosynthates toward reproductive growth, under drip irrigation than surface irrigation, as earlier reported in citrus (Panigrahi et al. 2012). Moreover, the higher fruit yield was due to the higher number of fruits under ISD than IS. Better flowering accompanied by reduced flower and fruit drops (104 No.) resulted in higher on-tree fruit storage in CTPR + RWHT + ISD than the other treatments. Better plant physiological activity under optimum soil moisture content and nutrient availability might reduce the fall of flowers and fruits of citrus plants under CTPR + RWHT + ISD. The lowest fruit yield (7.14 t ha⁻¹) was observed in RFT due to the lowest number of fruits and fruit weight in this treatment. The highest WP (3.47 kg m⁻³) was found in CTPR + RWHT + ISD, followed by CTPR + RWHT + IS (2.71 kg m⁻³). Despite higher water use, a higher fruit yield resulted in higher WP in CTPR + RWHT + ISD than the other treatments. It indicates that the percentage of increase in yield was higher than the percentage of increase in water use in CTPR + RWHT + ISD compared to the other treatments. The RFT produced the lowest WP (1.88 kg m⁻³) among the treatments. Earlier, it was also observed that the WP could be increased up to 72% under the conservation of rainwater in citrus orchards, due to the improvement in yield (Panigrahi et al. 2008; Kumar et al. 2015).

The highest SYI was observed under CTPR + RWHT + ISD (0.88), followed by CTPR + RWHT + IS (0.79). The lowest value (0.59) of SYI was observed in the RFT, reflecting the higher sustainability of citrus production under conservation measures in the study region. The EUE under different treatments followed the same trend of SYI in citrus orchards. Earlier, Jat et al. (2005) also found higher EUE under water conservation techniques in maize crops in a semi-arid climate.

The gross income (GI), net income (NI), benefit–cost ratio (BCR), gross economic water productivity (GEWP), and net economic water productivity (NEWP) in various conservation treatments and rain-fed treatment are given in Table 3. CTPR + RWHT + ISD produced the highest (310,216 INR ha⁻¹) annual GI, followed by CTPR + RWHT + IS (310,216 INR ha⁻¹), due to higher fruit yield in these treatments in comparison to other treatments. The NI (237,916 INR ha⁻¹) generated in CTPR + RWHT + ISD was also the highest, in spite of the higher cost of production (310,000 INR ha⁻¹) caused due to additional expenses (310,000 INR ha⁻¹) done in the construction of water harvesting structure and installation of solar-powered drip irrigation system in this treatment. The increase in NI is attributed to a relatively higher enhancement in GI than that of the cost of production in CTPR + RWHT + ISD compared to the other treatments. The GI and NI in RFT were the lowest due to the lower fruit yield in this treatment. The BCR in various treatments followed the same trend as that of the NI. The highest gross economic water productivity (GEWP: 47.25 INR m⁻³) and the net economic water productivity (NEWP: 36.24 INR m⁻³) were observed in CTPR + RWHT + I_D followed by CTPR + RWHT + IS (GEWP: 36.99 INRm⁻³; NEWP: 27.30 INR m⁻³). A higher GI and NI obtained under CTPR + RWHT + I_D produced a higher GEWP and NEWP in this treatment than in other treatments.

In situ rain water conservation through continuous trenches made between plant rows substantially reduced the runoff in citrus orchards on high clay content black soil in central India. The inclusion of water harvesting tanks with continuous trenches completely harvested rainfall–runoff in citrus orchards. Rainwater conservation through continuous trenches and recycling of harvested rainwater in tanks significantly improved soil moisture in crop root zones, which boosted fruit yield and water productivity in orchards. Solar-powered drip irrigation helped in producing more fruit yield with less water, without using conventional energy, compared with surface irrigation. In spite of higher initial investment, the combined use of continuous trenches, rain water harvesting tanks, and solar power drip irrigation was found superior to other conservation measures in relation to fruit yield, water productivity, financial return, sustainability, and energy use efficiency in citrus production. The empirical models developed to predict runoff from rainfall data are helpful in planning and designing water harvesting structures in the citrus groves of central India. Overall, the study demonstrates that citrus cultivation under rainwater conservation using continuous trenches and water harvesting tanks, and recycling of harvested water in orchards through solar-powered drip irrigation, are sustainable and profitable options for the rain-fed areas in central India or regions with similar agro-climatic conditions as that of the study site. The technique will enhance the productivity and sustainability of citrus cultivation. Further research on effective and low-cost lining materials to reduce seepage and evaporation from water harvesting tanks is suggested.

The author acknowledges the support and facilities given by the Director, ICAR-Central Citrus Research Institute, Nagpur, India for conducting this study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.