Assessing the Effect of Different Water Table Depths on Water Use, Yield and Water Productivity of the Okra Crop

An experimental study was carried out on a Lysimeter with the aim of partially meeting WRs (Water Requirements) of the Okra crop through SWT (Shallow Water Table) while maintaining the SWT at various levels below the ground surface. Under the study, CWR (Crop Water Requirement), yield, water productivity, salt accrual and contribution of SWTs towards meeting the CWR are assessed. The study was designed in accordance with the principles of CRD (Complete Randomized Design) with three treatments and four replications. The treatments; viz. T 1 , T 2 , and T 3 consisted of maintaining the WTDs (Water Table Depths) at 45, 60 and 75 cm, respectively, below the ground surface. The crop was irrigated with a good quality water having EC w = 0.50 dS m -1 and pH = 7.3. The results of the study showed that the crop consumed the maximum amount of water under T 1 treatment, followed by T 2 and then by T 3 treatment. Accordingly, the contribution of SWTs towards the CWR is 94.8, 93.2 and 42.9% of the total CWR under the T 1 , T 2 , and T 3 treatments, respectively. Maximum yield is attained under T 3 treatment, followed by T 2 treatment and then by T 1 treatment. Likewise, maximum water productivity is achieved under T 3 treatment, followed by T 2 treatment and then by T 1 treatment. The dry bulk density ( ρρρρρ d ) of the soil, under T 1 and T 2 treatments, increased slightly; however, it remained unchanged under the T 3 treatment. The EC se (Electrical Conductivity) of the soil increased, whereas, the pH value of the soil decreased under all the treatments. Statistically, significant difference (p < 0.05) is observed in CWR, yield, water productivity, contribution of SWT towards crop water use, plant height and weight of the Okra pod; whereas, the difference in ρρρρρ d , EC se , pH and length of the pod is observed as not significant (p > 0.05) under the three treatments. Accordingly, to make profitable use of SWTs, improve WUE and productivity, and maintain soil fertility, the depth of SWT be controlled at 75 cm for growing of the Okra crop. Adapting to this guideline will help in availing the maximum contribution of SWTs towards meeting the CWR and achieve the larger aim of water conservation.


INTRODUCTION
water supply and demand is also widening with every passing day. Population increase is pressing for additional food and domestic water supplies, consumer goods and W ith increasing population, urbanization and industrialization, competition for water is increasing worldwide. The gap between water for environmental needs from the existing water resources. Worldwide, more than 40% of food production depends on supplements by irrigation waters [1]. In Pakistan, agriculture consumes more than 93% of good quality water [2,3]. Due to increasing agricultural activities in the country, the gap between water supply and demand has widened manifolds. Thus, there is a dire need of fresh water resources to meet the ever-increasing demand of various users. Given the scenario of not having much potential for additional water resources to be developed, the only option remains to manage the available water resources sensibly and adapt to best management practices.
Since, agriculture is the major consumer of fresh water resources, any effort towards improving WUE in this sector will be worthwhile. Increasing WUE through improved irrigation technologies, crops, and improved productivity of lands by maintaining soil fertility will be a complimentary approach towards managing irrigation waters and saving waters for other logical uses. Currently, throughout the world, the average irrigation efficiency, resulting from surface irrigation techniques, range from 30-50%. Poor irrigation efficiency offers an opportunity for improvement that will result in additional water supply for cultivating extra lands or for other uses; however, this must not be at the cost of negatively impacting yields.

SGW (Shallow Groundwater) is an important resource that
can be harnessed to irrigate crops through subsurface irrigation technique and thereby reduce the gap between water supply and demand [4]. Depending on the quality of SGW and type of the crop grown, this technique can be used to meet fully or partially the WR of crops. So far, numerous studies have been conducted regarding the use of SGW as an additional source of irrigation water [5]. Using the SGW resource to supplement the crop water demand is affected by the level of WTD, groundwater quality, crop growth stage, crop salt tolerance, irrigation frequency and application depth, and whether the crop is seasonal or perennial. Literature reveals that various investigators have investigated the impact of SWTs on crop yield, land and water environment, water saving and crop water productivity.
Nosetto et. al. [6] reported that, normally, the rise of the WT and its getting into the root-zone profile has adverse impacts on crop health and thereby on yield as well.
According to Liu and Luo [7], the problem of waterlogging induced by SWTs can be addressed by curtailingdown the amount of irrigation waters, and instead using the SWT to meet the crop water demand; however, they suggested that maintaining the WT at optimum level is essential to make its profitable use without degrading the soil environment. Karimov et. al. [8] while using HYDRUS-1D numerically simulated the groundwater contribution towards the ET (Evapotranspiration) need of wheat crop and also the salt concentration in the root-zone profile; they found that SWTs increased the salt concentration within the root-zone profile. They argued that groundwater potential can be favorably used for subsurface irrigation, which will also help in lowering down the WTDs and help in to achieve water saving if proper cropping pattern and WTDs are managed.
Mejia et. al. [9] obtained maximum yield for corn and soybean crops under artificially controlled WTDs at 0.5 and 0.75 m in comparison to free drainage treatment, in which WT level was > 1.0 m below the ground surface.
Khan et al. [10] observed that the sugarcane yield was highest if the WTD ranged between 1.2-1.9 m, and the yield reduced if the WTD was shallower than 1.0 m or deeper than 2.0 m from the ground surface. Nosetto et. al. [6] found that when the WTD ranged from 1.4-2.45, 1.2-2.2 and 0.7-1.65 m; the maize, soybean, and wheat crops resulted in maximum yield, respectively; they also concluded that crop yield decreases if the WTD is shallower than these limits. Zhu et. al. [11] investigated the impact of WTD on soybean yield and concluded that the soybean yield reduced if WTD was shallower or deeper than 2 m.
Mueller et. al. [12] observed that highest WUE can be achieved by controlling the WTDs at the optimum level.
Kahlown et. al. [13] found that the wheat and sunflower used 100 and 80% of their water consumption, respectively, through subsurface irrigation when WTD was maintained at 0.5 m. Liu and Luo [7] observed that the wheat used 65% of its WR from the groundwater if the WTD is maintained between 0.4-1.5 m; moreover, they also observed that the surface and subsurface water productivities are enhanced and these are proportional to decrease in WTD. Huo et. al. [14] found that the wheat crop consumed 29% of its water demand from subsurface waters when the WTD is maintained at 1.5 m. They also observed that the contribution of subsurface irrigation helps in lowering down the WT. Ayars et. al. [1] while reviewing the feasibility of groundwater use in irrigated agriculture argued that SGW of good quality can be exploited and managed for crop water needs with the predominant factor of proper crop selection. Xu et. al. [15] found that if the WTD is shallower than the 1.0 m, then the salt concentration in root-zone profile increases thereby yields decrease; for optimum growth of wheat, they suggested that WTD be maintained between 1.0-1.  [16]. Due to flatter topography of the area, the rise of the river bed, use of conventional irrigation techniques, high temperatures, and meager rainfall, the province is suffering from twin problems of water-logging and salinity. Of the total area of province, about 30% is waterlogged and saline, which poses threat to food security and sustainability of agriculture [17]. In 1999, about 2.2 Mha of the land has WTD ranging between 0-1.5 m below the ground surface; in the year 2001, the area then drastically reduced to about 0.26 Mha due to drought conditions in the province. In 2003, the area under SWT again rose to 2.7 Mha and since then it is continuously rising [18]. It's worthwhile to note that, in the area, before the inception of irrigation network, the WT remained deeper than 30 m. However, after the introduction of canal irrigation system, the WT has risen-up significantly and at present, it varies between 1.5-9 m below the ground surface [19]. According to Kahlown and Azam [20], normally in Sindh province, the WTD remains within 1.5 m range in an area of about 1.62-2.03 Mha of the irrigated land. Basharat [21] reported that the water-logged area in the province ranges from 1.5-3.5 Mha. Thus, in the light of the statistics presented above, there lies a great potential in the province to harness the SGW resources as a means of supplementing the CWR through subsurface irrigation method to grow various crops. However, this requires conducting research to determine the optimal WTD for any specific crop so that crop water productivity is maximized and environmental health of the soil is not endangered.
Therefore, studies to grow crops under SWTs need to be Okra (Hibiscus Esculentus L.) belongs to the family of Malvaceae [22]. Pakistan is the second largest producer of Okra, and in Sindh, this crop is grown throughout the year. It is an important source of vitamin A, B, and C and is also rich in protein, carbohydrates, fats, minerals, iron, and iodine. In Pakistan, this crop, locally known as 'Bhindi', is widely grown in Punjab and Sindh provinces.
It's estimated area under cultivation is about 2.21x10 5 ha, yielding about 2.86x10 6 Tons of green pods annually [23]; whereas the average world production of this crop is about 12.035 M Tons annually [24].

Site Description and Experimental Design
The study was conducted at the DRIP (Drainage and

Description of the Lysimeter
The  This water was applied just to allow germination of the seed. Later, no any irrigation water was added to T 1 and T 2 treatments through surface irrigation technique for the entire crop period, except in the case of T 3 treatment.
Throughout the cropping period, soil moisture content was monitored and thereby irrigation water was scheduled. For this purpose, a tensiometer was installed in each chamber; its penetration depth was changed with stage of the crop growth. The penetration depths were kept at 15, 30 and 45 cm up till 25 th , 55 th and 110 th day of the sowing period of the crop, respectively. Fig. 4 shows a picture of Lysimeter chambers and tensiometers at the site. When the soil moisture tension reached to 70 kPa, irrigation water was added to T 3 treatment through surface technique [26] and for each irrigation exercise; 75 mm (11.85 liters) of water was applied. Any surplus water (drainage effluent) was collected in Jarry cans through the drainage outlets provided at the bottom of each Lysimeter chamber.
The design WTDs of 45, 60 and 75 cm below the soil surface were maintained artificially in each Lysimeter chamber through mariotte bottles (Fig. 1). The water productivity of Okra crop under different treatments was also worked out using the following relationship [28]: where, CW P is crop water productivity (kg m -3 ), Y represents the yield of the crop (kg ha -1 ) and WR-denotes the amount of total water consumption for crop production (m 3 ha -1 ).
Agronomical parameters such as plant height, length, and weight of the pod were also measured and recorded under different WTD conditions [29]. As the crop reached to fruiting and marketable stage, the green pods of the crop were picked. All the collected data was statistically analyzed using the software, the Statistics 8.1.  Xuet al. [15]. These investigators found that when the WTD was shallower than a certain specific level, salt concentration increased within the root-zone profile with the contributions for CWR made from SWTs. From Table   1, it can also be seen that, after the conduct of experiment, the average soil pH value reduced by 0.2, 0.1 and 0.1 under T 1 , T 2 and T 3 treatments, respectively. From Table   1, it can also be seen that the average soil ρ d , under T 1 and T 2 treatments, increased by 0.02 and 0.01 g cm -3 , respectively, but, it remained unchanged under T 3 treatment. Obviously, the maximum increase in the density occurred under T 1 treatment. As a result of increase in salt concentration under the two treatments, T 1 and T 2 , the ρ d value in these treatments also increased.

Soil Properties
Nevertheless, the differences in soil EC se , pH and ρ d are not significant (P > 0.05).

Water Productivity
The water productivity of the Okra crop under T 1 , T 2 , and T 3 treatments are 0.63, 0.84 and 1.48 kg m -3 , respectively (Fig. 7). Because of higher yield and minimum amount of water consumed by crop, higher water productivity are obtained with deeper WTDs. Huo et. al. [14], Liu and Luo [7] and Mueller et. al. [12] also support these findings.
Analysis of variance shows that there is a highly significant difference (P < 0.05) in water productivity of Okra crop under different WTDs.

Agronomical Parameters
During the study, agronomical parameters of the crop, such as the height of the plant, length, and weight of the Okra pod were also examined. al. [6]. There were significant differences (P < 0.05) in plant height and weight, and non-significant difference