Determination of water requirement and crop coefficient for strawberry using lysimeter experiment in a semi-arid climate

Water is one of the most vital resources required by human society and the preservation and exploitation of which are thought to be among the principal challenges in the current century (Miguel Costa et al. 2007). Weighing up the crop water requirements for the design and implementation of soil and water management policies seems indispensable (Igbadun 2012; Sedri et al. 2019). Water requirement is referred to as the total amount of water consumed through evapotranspiration (Hafizul Islam et al. 2016). Iran is a semi-arid country which faced severe drought in recent years (Amini et al. 2014). Moreover, agricultural withdrawals for irrigation have caused an intense decline in the water resources in the region (Hesami & Amini 2016). Therefore, the minimization of agricultural water consumption in this area has been of particular interest to decision-makers and stakeholders. Inadequate optimal use of irrigation water, water constraint, and the increasing human need for more food dictate modern management practices to save water and increase irrigation efficiency. This is not feasible without an estimation of the crops’ water requirement (El-Nady & Borham 2009).

Thanks to its many comparative advantages, the strawberry is one of the most important small fruits in the world that enjoys great nutritional value. According to the Food and Agriculture Organization (FAO 2018), the total area of world strawberry cultivation in 2018 is 372,361 ha and the total production across the globe is over 8 million tons. Strawberry is a shallow-root plant that is very sensitive to water and mineral deficiency (Kruger et al. 1999). The yield and growth of strawberries are severely abated during flowering and fruiting periods under dehydration and drought stresses (Blatt 1984). The vegetative and reproductive growth of strawberries is very sensitive to environmental circumstances. This plant grows with the increase in day length, appropriate temperature, and nutrients. All of these are quantitative reactions so that different varieties are the results of different feedbacks (Sønsteby & Heide 2006). Different strawberry varieties grow in relatively wide climates. Certain abiotic factors such as water logging, drought, salinity, soil acidity reaction, frostbite, and air temperature may cause fertility cutback (Hancock et al. 1991). Permeability and high moisture-holding capacity, light texture and appropriate depth, relatively low salt level, being free of harmful weeds, and pH range of 5.8–6.5 are all the specifications of a soil apt for planting strawberries (Sharma 2002). Perea et al. (2016) developed software to facilitate irrigation scheduling in the strawberry sector. This tool uses soil data, agro-climatic, and hydraulic information and offers farmers information about the required irrigation.

García-Tejero et al. (2018) estimated strawberry crop coefficients (K_c) using in situ data from three lysimeters for 2 years. They recorded agro-climatic information from weather stations located inside and outside of the macro-tunnels. They found that K_c values estimation depends on the methodology used to estimate ET₀ and irrigation water requirements depending on the climatic conditions. Harfoush & Jumaah (2020) conducted a field experiment to estimate the strawberry K_c and water consumption. They found different values of water consumption under various irrigation standards and soil characteristics. The values of K_c were found from 0.35 to 0.75.

Lysimeter is applied to the measurement of evapotranspiration, evaluation of the crop water requirement, and crop coefficient aimed at efficiency improvement (Marek et al. 1988; Bergström 1990; Young et al. 1996; Yang et al. 2010). The noteworthy effects of irrigation on strawberry growth, berry yield, and quality were reported in previous research works (Serrano et al. 1992; Kruger et al. 1999). Liu et al. (2007) used a field lysimeter to study the effects of partial root-zone drying on strawberry irrigation water use efficiency, berry yield, and yield components. They found that given the same amount of irrigation water, partial root-zone drying had no advantage compared to deficit irrigation in maintaining plant water status and berries yield. Moreover, they found that a reduction in irrigation water led to significant reductions in berries yield. Using soil and water balance for irrigation management entails the estimation of the amount of water in the crop's development area (Li et al. 2010). Determination of water requirement for strawberries is a requisite for countries in a semi-arid area. In Iran, most strawberry is traditionally cultivated on open-field systems. In recent years, protected cultivation has also been developed in greenhouses and under plastic tunnels. Most family members are involved in its production, creating high-value goods and employment.

A great deal of research has been carried out thus far around the estimation of the crop water requirement. With due regard to the above-mentioned issues, it is essential to have efficient irrigation to achieve optimal crop quality (Fallahi et al. 2010). Despite the widespread use of drip irrigation, there is still indecision about the accurate amount of strawberry water requirement (Lozano et al. 2016). The objective of this study was to measure evapotranspiration, crop coefficients, and crown coverage of strawberries in a semi-arid area using the volumetric lysimeter installed in the field. The main innovation of this research is to measure evapotranspiration in a direct way and determine the characteristics of strawberries in a semi-arid climate. This study is a major step toward managing limited water resources in semi-arid regions and is the first attempt to determine the characteristics associated with strawberry growth and water use. The results will directly affect the livelihoods of a large proportion of farmers in the region.

Similar to the strawberry crop pattern in Kurdistan Province, Queen Elisa which is a short-day variety and the dominant cultivar in the region was used. The strawberry transplants in a double row-raised bed system were equipped with drip irrigation. Analysis of soil physicochemical properties was performed at the cultivation site. The operations of soil preparation including the distribution of rotten manure and fertilizers were run based on soil test results. The same transplants were arranged and planted on three stacks inside the lysimeter. The raised bed center spacing was 120 cm and strawberry transplants were planted out in two rows at a distance of 35 cm on a raised bed, 60 cm wide, and 20 cm high so there were 18 strawberry plants in two rows of nine in each experimental unit. Agronomic care including weed control was conducted in several stages along with regular interval irrigation as measured by soil moisture content. Soil moisture status was measured per lysimeter by Time-Domain Reflectometry, TDR, three times a week at 15 cm depth between the drip tapes and rows. Depending on the weather conditions, irrigation was done two to three times a week, as recommended by Létourneau & Caron (2019) at the time when soil suction reached −10 kPa so that it is kept within the readily available water, RAW, level (Allen et al. 1998). To attain RAW, the field capacity, FC, and moisture are to be determined. To measure FC, the soil inside the lysimeters was first saturated, afterwards, water was allowed to infiltrate for 24 h. Once the amount of inlet and outlet water was balanced, the rate of FC was attained. After establishing the moisture balance, the moisture percentage of the samples was measured using an oven. Based on the soil moisture curve of the test area, the percentage of gravimetric soil moisture at the FC and wilting points were 27.3 and 13.1%, respectively. Considering the soil texture, these values are in the ranges recommended by Allen et al. (1998) and Rai et al. (2017).

The irrigation system was run as drip tape in both lysimeters, and the inlet water measurement was rendered by a flowmeter with 0.1 l/s accuracies. The total water consumption in liters was calculated for each irrigation by this flowmeter and based on the duration of irrigation. The flow meter was installed at the beginning of the irrigation system.

The TDR instrument was employed to measure soil moisture. To log the captured data, the data logger of the TDR model SMS-T1IDRG was used. This device is capable of saving 2,000 temperature and moisture data in its memory and can monitor and record data independently of or connected to the computer. The irrigation and drainage systems were installed according to the offered standards (Yang et al. 2010). The device probes were positioned vertically at depth of 15 and the environs of which completely covered with soil. In certain cases, soil moisture content was calculated by the gravimetric method through laboratory sampling. The device possesses a microcontroller core, non-volatile memory, and other peripherals.

The soil samples of the lysimeters were taken from 0 to 30 cm depth and soil characteristics were analyzed based on the methods recommended by Hazelton & Murphy (2016). Also, to measure the quality of the water used, water samples were taken from the well and went into analysis in the laboratory of the Soil and Water Research Department of AREEO. The physicochemical properties of the soil sample are given in Table 1.

Table 1 shows that the soil at the experiment site is relatively calcareous with an alkaline pH, and while containing organic matter the absorbability of its phosphorus concentration is moderate and that of its potassium is high. The soil texture is sandy clay loam and lacks salinity which is a positive issue since strawberry is very sensitive to salty soil. According to the results, nutrient requirements for strawberries including nitrogen and phosphorus were determined, respectively from urea (%46 N) and triple superphosphate (TSP) fertilizers and added to the soil before cultivation at 15 cm depth. Nitrogen fertilizer was used in the amount of 100 kg of pure nitrogen per hectare from the source of urea fertilizer in three equal installments before planting the strawberry plant, 2 weeks before flowering, and after the first fruit harvest. Phosphorous fertilizer from the source of TSP fertilizer at the rate of 75 kg P₂O₅/ha was placed at a depth of 8 cm in the soil and at a distance of 5 cm from the plant next to the crop line.

The 10-cm pipes were set on the floor and at the side walls of the lysimeters to drain water out. The excess water in both lysimeters was directed to two graduated cylindrical containers. After filling the containers, the total volume of water was discharged by a graduated vessel and its volume was calculated. Thus, following each irrigation or precipitation, the drain volume was measured in liters and then converted to a value in millimeters by dividing it by the lysimeter's area to water depth.

Reference crop evapotranspiration is defined as the evapotranspiration from an unlimited area of grass with a uniform height of 8–15 cm, with an active growth without any water scarcity, nutrients, air and pests and diseases that cover the entire ground surface with its shade (Bhantana & Lazarovitch 2010). The reference crop is referred to by FAO as having 12 cm height and surface resistance of 70 s/m with Albedo coefficient-light reflection capability (Allen et al. 1998). Being a climate-related profile, the notion of reference crop evapotranspiration is discussed to examine the evaporation rate in the atmosphere regardless of the plant type parameters, growth stage, and crop management operations. Due to irregular recording of radiation, air temperature, air humidity, and wind speed data at the synoptic station near the study area, direct calculation of ET₀ and crop evapotranspiration (ET_c) was used based on the guidelines provided by FAO 56 in the lysimeters in 2018 and 2019. In the present research, evapotranspiration of the reference crop and strawberry was estimated directly by the cultivation of these plants in the lysimeters in 2018 and 2019.

To measure the precipitation rate during the growing season, an automatic rain gauge was set up in the research area. Moreover, the precipitation data were received from the meteorological stations located in the Sanandaj Synoptic Station around 3 km from the research area. These data were used in the calculation of water balance in the lysimeters.

Factors affecting crop coefficient are classified as plant type, climate, evaporation status of soil surface, and stages of plant growth (Grattan et al. 1998).

The percentage of canopy coverage was measured at different stages of plant growth according to the method suggested by Grattan et al. (1998). For this purpose, the diameter of the canopy cover was measured every 15 days in the lysimeter at noon and the average shading area of each plant was figured out. To determine the percentage of canopy cover per unit area, the average canopy diameter of a plant was multiplied by the number of plants per unit area (Grattan et al. 1998). As recommended by Cahn (2015) the regression relationship between the canopy cover percentage and the crop coefficient of strawberry as well as the concerning function was determined by MS-Excel 2016.

The rate of water requirement, as well as the characteristics of strawberry growth in normal circumstances, was investigated. Hence, the irrigation interval was so adopted that no stress was applied to the plant. Table 2 contains the results of evapotranspiration measurements and the irrigation rate in a 15-day time interval as recommended by Lozano et al. (2016).

Table 2 shows that during 240 days, as the growing season, the rates of evapotranspiration and irrigation water in the grass crop in 2018 were 1,108 and 994.8 mm, respectively. These values in 2019 were 1,179 and 1,037 mm. In 2018, these amounts for the strawberry crop were 878.7 and 848.7 mm, and in 2019 were equal to 932.1 and 898.6 mm, respectively. In previous works, a wide range of consumptive water, as 300 mm (Trout & Gartung 2004) and 797 mm (Strand 2008), has been reported. There are discrepancies between the consumptive water rate obtained in this investigation and that reported by Sahraee et al. (2015) and Lozano et al. (2016) for strawberry which is owing to differences in the type of cultivation system, the cultivar type, and the conditions of the site of the experiments. It is worth noting that Lozano et al. (2016) and Sahraee et al. (2015) have used, respectively, plastic tunnels and a greenhouse for the cultivation of strawberry. Long-term data analysis from 1977 to 2014 from the Synoptic Station shows 3.58 mm/day evapotranspiration for Sanandaj city (Heydari & Heidari 2019). Based on data presented in Heydari & Heidari (2019), evapotranspiration for the growth period of strawberry was calculated as 1,114.5 mm which is in agreement with the results of this research. In addition, examining the long-term changes of evapotranspiration shows its slow increasing trend in spring and summer (Heydari & Heidari 2019).

As shown in Figure 5, the reference crop evapotranspiration changes versus the planting time first follow an irregular increasing trend up till the lapse of the sixth 15-day period, i.e., 90 days following the beginning of cultivation. Then, in the second half of August, it reaches the maximum value. With the onset of September and temperature decline, the evapotranspiration shows a downward development. Considering the data obtained in two growth years, the seasonal average evapotranspiration, ET₀, was 1,143.5 mm and its daily average was 4.7 mm/day. The maximum and minimum values of ET₀ were attained in the second half of August and the first half of April respectively. As expected typically, the plant's demand for water is higher in the warm seasons. With due regard to the fact that the end of the time interval corresponds to the autumn months, the decreasing trend of evapotranspiration in the last days is more justifiable. The evapotranspiration measured in Kurdistan Province quite agrees with that obtained in the surveys conducted by Faghih (2015) and Sahraee et al. (2015). For instance, the most evapotranspiration for the grass crop as given by Sahraee et al. (2015) is rather less than 10 mm/day which conforms to the maximum value estimated in this research. As held by Allen et al. (1998), the utmost evapotranspiration reported by FAO 56 occurred on the 230th day. Since a 15-day period was considered in this research, the highest grass evapotranspiration happened in the 11th period, that is, 170 days later. Comparatively, the results of this research indicate a 50-day period difference. By the way, the evapotranspiration rates in both investigations comply with each other. It should be noted that in the case of Kurdistan province, the analysis of long-term data confirms that rainfall and temperature have experienced changes so that rainfall decrease and temperature increase mostly occurred in the months of March and February (Amini 2020). Therefore, the data of this research may need to be modified according to climate changes in the coming years.

Referring to Figure 6, one observes a mild slope in the evapotranspiration changes at the beginning of the growth period. With the commencement of the heating season, the evapotranspiration process would experience a momentous upward trend and would soar to its top value in the first half of August. The daily average evapotranspiration of strawberries in the whole crop season of 2018 and 2019 were 3.7 and 3.9 mm/day, respectively, and the average of both years was 3.8 mm/day. The most evapotranspiration happened in the first half of August, and the least value took place in the second half of November. The higher evapotranspiration rate of strawberries late in the time interval relative to the grass is a token of the water requirement of the former in this period. The results of the current research were compared to those of Sahraee et al. (2015) to the conclusion that both surveys share similar maximum evapotranspiration values. Incidentally, the maximum value reported by Sahraee et al. (2015) has occurred 92 days following the start of the experiment (the growth period), whereas that for the present research emerged about 150 days after cultivation. The difference is due to how Sahraee et al. (2015) conducted their tests in the greenhouse conditions.

The diagram of K_c alterations as well as strawberry canopy cover in various growth phases are depicted in Figure 7.

In this study, the growth and evapotranspiration of strawberry and grass plants were measured by field measurement using two volumetric lysimeters in 2018 and 2019. The most important results of this research can be stated as follows.

• The average evapotranspiration of the strawberry plant and the total consumed water in the whole period were, respectively, as 905.4 and 873.5 mm. The daily average for both years was found as 3.8 mm/day. The maximum rate of strawberry evapotranspiration occurred in the first half of August, and the minimum value corresponded to the second half of November.

• The total evapotranspiration of the grass plant during the growing period reached 1,143.5 mm with a daily average of 4.7 mm/day. The highest and lowest corresponding values as to the grass evapotranspiration were obtained to be 7.73 in 2019 and 0.03 mm/day in 2018 happening in the second half of August and the first half of April.

• The initial crop coefficient pertaining to strawberry was 0.45, with its maximum rate of 0.86, and the ultimate value as 0.8.

• During the growth period, the canopy coverage extended up to 73% of total area and then dropped to 65% in the late stages of growth. A positive linear relation between the changes in the canopy coverage and the crop coefficient was observed.

This work was conducted under financial support from The Ministry of Agriculture Jihad (Jahad-e-Keshaverzi Kurdistan) contract No. 37-1-108 and technical support from Soil and Water Research Institute, AREEO, research No. 24-53-10-034-961003.

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

The authors declare there is no conflict.