Here, we address the issue of water management for potato (Solanum tuberosum L.) crops under arid conditions in Saudi Arabia over a 5-year period (2011–2016). Daily evapotranspiration for irrigated potato crops (ETc) was determined over a period of three successive seasons. Non-weighing (drainage) lysimeters were used in four regions: Riyadh, Qassiem, Eastern Province, and Al-Jouf. Crop coefficients (Kc) were determined using lysimetric data and an alfalfa-based reference crop evapotranspiration (ETr) during each growing season. Measured (M-ETr) and predicted, (P-ETr) for different regions were compared. Comparisons between M-ETr and P-ETr show higher trends for the measured values in all regions, with averages of 5.72 mm/d, 4.42 mm/d, 5.06 mm/d, and 7.94 mm/d for the Riyadh, Qassiem, Eastern Province, and Al-Jouf regions, respectively. Differences in the ETr and ETc were also observed among the regions due to geographic and climatic variations. In addition, the water-use efficiency of potato crops was generally highest (5.45 kg m-3) in Riyadh and lowest (1.54 kg m-3) in Qassiem. Analysis of projected net returns for irrigated potato production revealed that greater economic potential exists.

Keywords: crop coefficient, lysimeter, penman-montieth, reference evapotranspiration, water economic return

The Kingdom of Saudi Arabia (KSA) is currently 121% self-sufficient in potatoes.¹ In 2010, the country’s potato crop encompassed a land area of 6,941 ha, with production of approximately 176,077 tons.² The successful irrigation of potato requires an understanding of both irrigation and scheduling methods, as a regular water supply is needed to secure high yields.³ Potato is often considered to be a high-water-use crop,⁴ with yield quantity greatly influenced by the amount of applied water.^5,6 In agriculture, quantifying evapotranspiration (ET) is essential for effective irrigation management.⁷ In the KSA, water requirements for potato crops average around 590 mm and 710 mm in the autumn and spring seasons, respectively.⁸ In eastern Saudi Arabia, Al-Omran and Shelby⁹ estimated the total crop water requirement of potatoes to be approximately 520 mm, whereas Al-Ghobari¹⁰ calculated a water requirement of 416 mm for the winter season in Najran. In contrast, Wright and Stark reported seasonal water consumption values in irrigated areas of Oregon and Washington between 640 mm and 700 mm, demonstrating the variation that exists in water use depending on factors such as environment, season, and rate of crop growth. Lysimeter is generally consider as the criterion system for estimating crop evapotranspiration (ET_c) and crop coefficients (K_c); as such, measurements of the water requirements of potato crops can be attained using lysimeters, which provide more reliable measurements of ET_c that, in turn, allows for more precise estimates of irrigation water depths.^11,12 ET_c measurements are used for different crops grown in Saudi Arabia,^13,14 and a standardized reference ET approach is commonly used to determine daily reference ET.¹⁵ The crop coefficient varies through the course of the growing season, depending on the growth stage of the crop.¹⁶

Moreover, it has been shown that K_c can vary within a crop due to differences in climate,¹⁷ cultivation practice,¹⁸ and surface situations.¹⁹ Published K_c values^20,21 typically derive from lysimeter-based environmental studies in which crops are grown under special climatic, irrigation, soil, and surface situations. The K_c and associated measurements of ET can be used to develop, calibrate, and test important ET-process models; as such, K_c values obtained from previous studies have been calibrated via experimental research with different pedoclimatic conditions by many researchers.^22–24 Alkaeed et al.,²⁵ emphasized the need for local calibration by adjusting K_c to match specific localized field conditions. Ferreira²⁶ have shown that the seasonal values of K_c for potatoes are 0.87-0.85, whereas Kashyap²⁷ estimated K_c values at different stages of potato crop development using measured ET_c. Crop coefficients (Kc) for potato have been documented significantly by previous studies to irrigate crops.^28–30 Carvalho et al.,¹² calculated K_c values of 0.35, 0.45, 1.29, and 0.63 for potato (Solanum tuberosum L.) at different stages of crop development. Fan et al.,³¹ calculated the water-use efficiency (WUE), ET, and yield (Y) for crops through statistical analyses of the data, whereas Salvador et al.,³² applied WUE and water productivity indicators to evaluate irrigation performance, as well as the variance among irrigation methods and yields in the Ebro basin of Spain. Likewise, Andrés et al.,³³ used water productivity measure of the rule on both water quality and irrigation practices of the study area. In contrast, several researchers, including,^34,35 and Tarjuelo et al.,³⁶ have used economic models to optimize land use and water allocation to crops at the farm level and Pereira et al.,³⁷ developed deficit irrigation strategy, according to the estimates of economic indicators for the productivity and water use efficiency, taking into account the full awareness of the crop water requirements.  The objectives of this paper were to 1) estimate the water requirements for potato crops and the WUE in four regions of Saudi Arabia, and 2) assess the potential economic returns of potato production and potato water use.

The approach we adopted to conduct this study fell into five categories: 1) site locations, 2) lysimeter construction and operation, 3) crop cultivation and management, 4) prediction and measurement of alfalfa (as reference-crop ET, or ET_r) and potato ET_c, and 5) statistical and economic analyses.

Site location

Experiments designed to quantify the water requirement of potato crops were conducted in the four regions of the KSA where potato cultivation is most intense, namely Riyadh, Qassiem, Eastern Province, and Al-Jouf. The experimental sites were situated in the fields of agricultural research centers of the Ministry of Agriculture and in the fields of agricultural companies within each region (Table 1). Meteorological data for each region are presented in Table 2.

Lysimeters construction and operation

Four drainage-type lysimeters (non-weighing) consisting of compacted to nearly normal soil field conditions were constructed, each extending over a surface area of 10m×6 m and to a depth of 1.5 m. These were located in the middle of alfalfa fields in order to minimize the effect of advection at the field edges, and one lysimeter was allocated for each region. Each lysimeter consisted of nine plots (2m×2m×1.5m) and weighed approximately 15 Mg, with each plot containing a system of perforated pipes beneath the soil to facilitate drainage and polyethylene pipes laid out atop the soil to serve as surface irrigation systems. The irrigation systems included all necessary components, such as valves, water meters, and control boards, to ensure proper functioning. The lysimeter design and plot arrangement are shown in Figure 1. Tipping buckets were attached to the lysimeters for the purpose of collecting drainage water. The depths of the irrigation water added to the lysimeter, drainage water, and rainfall were monitored during the growing season. To improve soil properties, organic matter in the form of peat moss was added at a rate of 50 kg/plot in the upper soil layer to a depth of 15 cm. Initially, all plots were flooded with water to facilitate leaching of the soil profile and decomposition of organic matter.

Figure 1 Layout of lysimeter plots.

Crop cultivation and management

Potato crops

Winter potato crops of the Spunta variety (Solanum tuberosum L.) were randomly sown in three plots within each lysimeter in the four regions for three successive seasons (2011–2016), whereas the other three plots were planted with alfalfa throughout the study period to measure ET_r. The planting and harvesting dates of the potato crops in the four regions are shown in Table 3. The entire experimental period was of 5 years (2010–2015); tasks such as lysimeter construction and installation, and the preparation of the plots were carried out over the first 2 years, and soil samples were collected from the lysimeter plots prior to the commencement of the experiment for the purposes of mechanical and chemical soil analyses. Three soil samples were extracted at three different depths (20 cm, 30 cm, and 60 cm) to determine the soil physical properties, with soils in all plots characterized as sandy loam. Potato tubers were planted manually in three rows at intervals of 30 cm and a depth of 0.10 m, with ridge heights of 20 cm and ridges separated by 75 cm.

Fertilizers and pesticides

Fertilizers were added for the three cropping seasons in the same quantities and in accordance with the recommendations and schedules set out by the KSA Ministry of Agriculture. Nutrients N, P, and K were supplied at the rates of 200 kg/ha, 200 kg/ha, and 150 kg/ha, respectively. The P- and K-based fertilizers were utilized as basic doses prior to planting. Nitrogen was parted and distributed along with the irrigation water in all treatments for a period of early vegetative growth, and 100 kg/ha of potassium nitrate was applied to each plot following the tuber initiation stage. Pesticides were used to control insects and pathogens and applied at times and rates commensurate with the prevailing norms for the region. The fungicide Penncozeb 75 DG was applied at a rate of 2.5 g/L for all treatments in the four locations during the three seasons, and plants were sprayed with the pesticide Imidor 200SL at a rate of 5 ml/10 L for insect control.

Harvesting

Harvesting is a critical step in the potato production process and for future product marketing; the lysimeter plots in the four regions were harvested on different dates taking into account the specific conditions of each region over the three successive seasons. Potato production costs and sale prices were obtained from the Chamber of Farmer Association, and gross revenues were calculated by multiplying the first- and second-class yields and their prices. Finally, irrigation costs were calculated from data reported by Sener et al.,³⁸

Water-use efficiency

Water-use efficiency (WUE) is defined as the yield obtained per unit of water consumed, whether from irrigation or the total amount of water received, and therefore includes natural precipitation.³⁹ The WUE (in units of kg m^-3) was calculated using the method described by James⁴⁰:

$\text{W}\text{U}\text{E}=\frac{\text{Y}\text{(Kg}\text{/}\text{ha)}}{\text{A}\text{W}{\text{(m}}^{\text{3}}/ha/season)}$ (1)

Where Y represents the yield and AW represents the applied water during the growing season.

Predicted and measured ETr

Lysimeters were used to measure ET_r of alfalfa and ET_c of potato for the three consecutive growing seasons. Reference ET_r values were measured by the lysimeters for the growing seasons utilize the soil water equilibrium equation (2), as follows:

$E{T}_c=I+P-D-R\pm \Delta S$ (2)

Where ∆S is the change in soil-water storage (mm) between planting and harvesting dates, P represents the amount of precipitation (mm) recorded by the weather station at the site, I is the amount of applied irrigation (mm) as measured by a flow meter, and D is the amount of drainage (mm) from the bottom of the root zone. No surface runoff (R) occurred on the experimental sites during the study seasons, as rainfall intensity is usually low. The K_c incorporates crop characteristics and averaged effects of evaporation from the soil, which is the ratio of ET_c to ET_r, and also integrates the effects of the field environment, such as ground cover, canopy properties, and aerodynamic resistance. The potato crop ET_c relates to the reference evapotranspiration (ET_r) via the following equation:

$K_c=E{T}_c/E{T}_r$ (3)

Where K_c is the crop coefficient and ET_r is the measured reference evapotranspiration expressed in units of mm/d. The measured evapotranspiration for alfalfa (M-ET_r) during the years 2012-2015 was compared to the predicated evapotranspiration (P-ET_r) using the Penman-Monteith (PM) formula on a daily basis. The PM equation is a standardized estimate of the ET of 50-cm tall alfalfa; to perform the calculation, an automatic weather station was installed beside each lysimeter, from which the input parameters (temperature, relative humidity, radiation, wind velocity) for calculating alfalfa ET_r using the PM formula were collected.

Statistical and economic analysis

The experiments used a split-plot design with three replications. Analysis of variance (ANOVA), carried out by using the SAS version 9.1 computer program.⁴¹ That allowed evaluating the statistical differences between treatments in terms of locations were assigned to the main plots and crop species were distributed randomly over the sub-plots. ANOVA was assessed according to Steel⁴² and Torrie. Economic analysis was performed using the prevailing market prices for inputs at planting and for outputs at harvest. The economic potential for irrigated potato production in the study regions was assessed from two angles: the estimated net return and the return from water use. Assessing the economic potential included estimation and comparison of average area productivities (ton ha^-1), water requirements (m^-3 ha^-1), water costs (US$ m^-3), market prices (US$ ton^-1), variable costs (US$ ha^-1), economic return (US$ ha^-1), net return (US$ ha^-1), and water return (US$ m^-3).

Analysis of measured and predicted ET

Averages of the monthly measured and predicted ET_c for potato crops during the three seasons for the four locations are shown in Figures 2 and Figures 3. Figure 2 shows the variation in measured ET_c among the regions and at different growth stages, ranging from a low of 2.1 mm/d in the Eastern Province during the germination stage to a high of 12.5 mm/d in the Al-Jouf region during the flowering stage. The average measured seasonal ET_c values were 5.72 mm/d, 4.42 mm/d, 5.06 mm/d, and 7.49 mm/d for the Riyadh, Qassiem, Eastern Province, and Al-Jouf regions, respectively. In general, peak ET_c for potato occurred nearly 8 weeks after sowing, and during the reproductive growth stage. Similarly, as shown in Figure 3, the predicted ET_c data for the same period ranges from 2.61 mm/d to 10.31 mm/d. The average M-ET_r values for the alfalfa crops were 4.52 mm/d, 4.86 mm/d, 5.00 mm/d, and 4.78 mm/d (Table 4), whereas the average P-ET_r values were 3.40 mm/d, 3.45 mm/d, 4.23 mm/d, and 3.66 mm/d for the Riyadh, Qassiem, Eastern Province, and Al-Jouf regions, respectively.

Figure 2 Measured ETc of potato for the four regions.

Figure 3 Predicted ETc of potato for the four regions.

Figure 4 Variation of the averages of the Kc for potato during the growth period over three seasons.

Statistical analysis

A goodness-of-fit analysis indicated that the variances among the means of the measured and calculated ET_r for the potato crops in Qassiem and Al-Jouf were significant, whereas no significant differences were found for the measured and predicted ET_c among the regions, implying that although there are differences in ET_r within each study region, ET_r is consistent between the study regions. Pearson correlation analysis indicated a powerful correlation among the means of the measured and calculated ET_r within each study region, ranging from a lower of 0.97 in the Qassiem region to a maximum of 1.0 in the other three regions, as illustrated in Table 5.

Potato crop coefficient

The predicted monthly K_c through the interval of study for all regions as calculated using Equation 3 is presented in Figure 4; variations in the averages of the coefficients related to the different crop development stages for the four regions over the three seasons are plotted in the same figure. Maximum measured K_c values for potato ranged between 1.12 and 1.16 for Al-Jouf and Eastern Province, respectively, whereas minimum average values ranged from 0.49 to 0.52 for the Al-Jouf and Eastern Province regions, respectively.

Water requirements

Gross irrigation water requirements during the growing period for the four regions based on ET_c lysimeter measurements under surface irrigation practices are shown in Table 6. The highest seasonal irrigation requirement was 11,843 m^-3 ha^-1 for Al-Qassiem, and the lowest value was 6,712 m^-3 ha^-1 for Riyadh. Seasonal WUE averages for the four regions are also shown in Table 6.

Water use efficiency

The WUE, which represents the relationship between crop yield and the quantity of water utilized, is presented in Figure 5. WUE can be used to ascertain the influences of the irrigation methods productivity of crops, which indicate the effects of different irrigation systems on potato crops. The highest WUE value (3.64) was measured for the Riyadh region, with the lowest observed in the Al Jouf region.

Figure 5 Water-use efficiencies of potato for the four regions.

Economic analysis

Economic data for irrigated potato production in the different regions over the 3-year period are shown in Table 7. The average water-return productivity (US$ ha^-1) was calculated by estimating the actual harvestable yield. It is clear from this table that the economic potential of irrigated potato production is highest in the Riyadh region, followed in order by the Qassiem, Eastern Province, and Al Jouf regions. The maximum average water return was also observed for the Riyadh region, with the minimum value occurring in Al Jouf. The average net return for the irrigated potato crop was estimated to be approximately US$4,601.30 ha^-1, US$7,426.90 ha^-1, US$865.30 ha^-1, and US$2,202.70 ha^-1 in Riyadh, Qassiem, Eastern Province, and Al Jouf, respectively. It is notable that the minimum water return over the three growing seasons occurred in Eastern Province, whereas the maximum occurred in Qassiem.

Analysis of measured and predicted ET

Figure 2, shows that there were significant variations in ET_c were detected between regions, most likely due to differences in their geographic location and topographies, especially elevation. In addition, meteorological conditions, such as radiation, wind speed, humidity, and temperature, greatly influence evapotranspiration. Therefore, it is necessary to reduce water losses due to increases in the evaporative demands in these arid regions, especially during critical periods. The differences in ET_c might be ascribed to daily variations in the maximum and minimum temperature resulting from different microclimates across the three seasons, and due to differing levels of relative humidity. The impact of climate variability on agricultural production is important at local, regional, national, and global scales.^43–45 The average monthly ET_c was relatively low in early growth stages but increased gradually, peaking during the flowering stage before dropping again at harvest. This might be due to ET characteristics during the initial stages, which are predominantly in the form of evaporation and affected by the evaporative power of the atmosphere, the magnitude of wetting events, and the time interval between wetting events. This result is in close agreement with that for the Eastern Province. Al-Omran and Shelby (1992) estimated total crop water requirements of potatoes at approximately 5.2 thousand m³/ha. Similar findings were also reported by Al-Ghobari¹⁰, who estimated a water requirement in Najran of approximately 4.16 thousand m³/ha for potatoes, which accords with the results of Sahin et al.,^28–30 Figure 3 shows that the average predicted seasonal ET_c rates of the four regions are similar to the measured values, with some minor divergences. Maximum ET_c occurred in the Eastern Province, with the minimum observed for the Riyadh region. The divergence between the measured and predicted values can be attributed to the PM equation, which does not take into account several micro-climatic conditions that may affect evaporation rates. Generally, however, the trends for the predicted values are similar to those of the measured values, and appear to be consistent throughout the season. Therefore, the ET_r is in a little undervalued nearly all sites, due to high evaporative request. This slight underestimate in the calculated ET_r may be due to the actuality that the system contains several experimental functions that require local calibration.

Potato crop coefficients

Variations in the crop coefficient relating to different crop development states for the four regions are shown in Figure 4. The results indicate that the average value of monthly K_c during each season is similar but not identical in all regions. This minor variation is justified given that the K_c consolidates crop properties and the means influence of evaporation from the soil. These averages are pertinent and extra appropriate than the K_c calculated on a daily time step utilize a separate crop and soil coefficient Characteristics.⁴⁶ Nevertheless, this variation is expected, given that K_c is affected by all factors influencing soil water status, such as the type and frequency of irrigation,^47–49 climate, soil characteristics, and agronomic practices.⁵⁰ Previous studies have shown that the interaction between management practices and climate influences K_c for initial, middle, and late stages of crop growth. In general, ET_r and the corresponding K_c are unique to a given crop, its microenvironment, and its geographic location.

Water requirements

As can be seen in Table 5, gross water requirement varied among the regions, which may be due to the differences in daily air temperatures and relative humidity, in addition to dissimilar irrigation efficiencies and management practices between the regions. A more comprehensive understanding of how water requirement relates to geographic location and micro-climates is needed.

Water use efficiency

Water-use efficiency was highest in the Riyadh region, most likely due to the higher yields achieved in this region and to the volume of irrigation water used, whereas WUE was lowest in Qassiem. Several factors affect crop yield in addition to the amount of water added, such as soil fertility, texture, and manure content. As such, our experiments clearly demonstrate the importance of the addition of appropriate amounts of irrigation water, as high levels of production can be attained even with reduced irrigation. Hence, there is an urgent need for improving crop WUE in order to ensure sustainable use of limited water resources.

Economic Analysis

The potential for an economically viable potato production industry is highest in the Qassiem and Riyadh regions (Table 6). Estimated net returns for potato production are influenced mainly by fluctuations in the average variable costs and the prevailing average market prices in the study regions, a trend that was similar between years. This is perhaps due to fluctuations in the demand and supply of tubers during the early growing season, with higher demand and lower availability driving up market prices. The average net-return ratio is a critical factor in crop selection and the choice of planting season; farmers in Riyadh, for instance, have switched to growing potato crops during mid-October in order to fetch a better return. Similar results were reported by Kumar et al.,⁵¹ for producers of okra, whereas Pereira et al.,³⁷ found that the performance of the proposed model, the estimated values of tuber yield was compared with observed productivity data under irrigation conditions for the studied sites.⁵²

This research was aimed for determination of exact plant water usage or crop evapotranspiration (ET_C) and crop coefficients (K_C) for potato crop. Here, we examined the ET_c of the potato Solanum tuberosum L. and the ET_r of alfalfa to determine the crop coefficient, K_c, of potato crops grown in four distinct geographic and climatic regions of Saudi Arabia with the aim of creating a framework for systematic scientific assessments of the water requirements of crops so as to avoid the underestimation or overestimation of irrigation requirements. A better understanding of appropriate levels of water use through measurements of actual evapotranspiration is necessary to achieve sustainable development and environmentally sound water management in arid regions such as Saudi Arabia. The irrigation requirements of potato were predicted using the PM model, with the average K_c values for the three growing seasons estimated to be 0.85, 0.83, 0.85, and 0.83 for the Riyadh, Qassiem, Eastern Province, and Al Jouf regions, respectively. The seasonal K_C values varied from 0.49 to 1.16 for potato. Our results presented that K_C values can be different from one region to the other. It is assumed that the different environmental conditions between regions allow variation in variety selection and crop developmental stage which affect K_C. We assume that the development of regionally based and growth stage specific K_C helps in irrigation management and provides precise water applications for this region. In conclusion the development of regionally based K_C helps tremendously in irrigation management and furthermore provides precise water applications in those areas where high irrigation efficiencies are achieved by surface irrigation. Therefore, from these results, it is concluded that the lysimeter method can be recommended for computing evapotranspiration (ET_c) and crop coefficients (K_c) for all areas in the case studies, measurements of the water, which provide more reliable measurements.

Water requirements for potato crops varied over the course of the growing season due to variations in geography and climatic conditions. The measured ET_c showed acceptable agreement with the data obtained from the lysimeters with a few exceptions; in many cases, corrections are unnecessary. Finally, our results emphasized the importance of efficient allocation of irrigated potato production to regions having comparative advantages in the Kingdom, such as Al Jouf and Qassiem. It is expected that adoption of a new openness in considering all costs and benefits, and more precise use of terms could lead to much improved. The consideration of economic water productivity indicators shows to be of relevance when analyzing improvements in irrigation systems and management at farm and project scales. Case studies are shown to illustrate the use of these economic water productivity indicators in practice. It may be expected that their adoption will provide for a better understanding of water conservation and saving. Inevitably this could lead to better understanding of water performance and productivity by farmers and the public and by government and water agency decision makers. However, further research is warranted using lysimeter method to optimize the irrigation strategies. This will lead to appreciable saving of water in potato production considering the large area it enjoys worldwide.

This Project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (11-AGR1476-02).

The author declares there is no conflict of interest.

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