Estimation of irrigation water requirement and irrigation scheduling for major crops using the CROPWAT model and climatic data

Soil, water, and plants are natural resources that are very important for the survival of humans and animals (Shah et al. 2022). These three resources must be managed scientifically and efficiently to achieve maximum food production and meet the needs of an ever-growing population. Throughout the growing season of a crop, a certain amount of water is required at certain predetermined intervals. Water is a fundamental factor in ensuring crop production. Water dissolves mineral nutrients that travel through the stem into the plant. At the end of a plant's life cycle, water is also a component of the economic product, which may be a seed, stem, leaf, flower, or fruit. In plant production, in tropical countries, the first two of the three basic needs of plant growth, heat and light are abundant, but the third, moisture, must often be supplemented by artificial water supply, i.e., irrigation. In the eyes of scientists and policymakers, freshwater scarcity is the second most important environmental problem of the 21st century after climate change (UNEP 1999). Currently, about 87% of consumptive water and 70% of global water withdrawals are used for irrigation purposes (Shiklomanov 1997). About 40–45% of the world's food is produced on irrigated agricultural land, which accounts for less than one-fifth of the total cultivated land. It is a fact that water demand for various purposes will increase in the future. Our sustainable solution to control the demand for water resources and the negative impact of irrigation on the environment is smart irrigation because a smart irrigation system reduces water consumption without affecting crop yields (Mason et al. 2019). Each crop has specific water requirements, so plants should only be provided the amount of water needed to maximize yield (Jamal et al. 2017). Plants that require little water are commonly referred to as drought-tolerant. They are able to grow under hot, dry conditions with very little water. In most cases, drought-tolerant plants are hand-watered because they require adequate water at seeding and during the growing season.

Plants need a certain amount of water to grow perfectly. If the water is not distributed according to the requirements, large water losses will occur. As a result, less area will be irrigated with a sufficient amount of water. Due to seepage from canals and over-irrigation, not only is a large amount of irrigation water loss, but it also leads to waterlogging. Farmers’ lack of knowledge about actual water needs is also a major obstacle. It is an undeniable fact that climatic parameters have a significant impact on crop and irrigation water requirements. Climatic conditions such as evapotranspiration and uneven rainfall distribution, soil fertility, and soil properties have a significant impact on crop water requirements (CWR) and consequently on the development of the country (Tellioglu & Konandreas 2017).

As an agricultural country, Pakistan is highly dependent on irrigation due to insufficient rainfall on agricultural lands in Pakistan. In the list of the world's largest sources of irrigation, Pakistan's irrigation system occupies a top position. It provides water to about 18 million hectares of arable land (Yasmeen 2021). Moreover, agriculture is the main source of income for about 72% of Pakistan's population. However, increasing demand for freshwater and conveyance losses from irrigation systems are likely to affect the water supply to agriculture.

In Pakistan, including the Hyderabad district, farmers usually over-irrigate their lands because they do not have sufficient knowledge of CWR and assume that more water will produce a higher yield. An appropriate irrigation strategy can reduce the negative effects of over-irrigation; on the other hand, a balance can be maintained between crop water demand and available water. Modeling the water requirements of irrigated agriculture as an adaptation to appropriate irrigation management is the need of the hour. It not only improves the understanding of the current water use but also facilitates the search for sustainable development paths for the future.

CLIMWAT is a climatic database that works with the computer application CROPWAT to calculate crop water requirements, irrigation water requirements (IWR), and irrigation scheduling for a variety of crops at various climatological stations across the world. CLIMWAT 2.0 for CROPWAT is a joint publication of the Water Development and Management Unit and the Climate Change and Bioenergy Unit of FAO. If local climatic data are not available, these can be obtained from CLIMWAT for over 5,000 stations in the world. CLIMWAT provides long-term monthly average values of seven climatic parameters, such as average daily maximum, minimum temperature, average relative humidity, average wind speed, average sunshine hours, average solar radiation, monthly rainfall, and effective rainfall (FAO 2018).

CROPWAT 8.0 for Windows is a computer program for calculating crop water requirements, and irrigation requirements based on soil, climate, and crop data. In addition, the program allows the development of irrigation schedules for different farming conditions and the calculation of water demand for different cropping patterns. CROPWAT can also be used to evaluate farmers’ irrigation practices and estimate crop performance under both rainfed and irrigated conditions (Feng et al. 2007).

Various researchers such as Solangi et al. (2022); Gabr & Fattouh (2021); Gebremariam et al. (2021); Akinbile (2020); Chaali et al. (2020); Moseki et al. (2019); Ewaid et al. (2019) have applied the CROPWAT model worldwide to efficiently investigate the CWR, IWR, as well as irrigation scheduling. The CROPWAT model is preferred in the determination of the reference evapotranspiration (ET_o), as it is reported to deliver very reliable values on actual crop water use data worldwide (Solangi et al. 2022). Therefore, the present study was carried out to model the CWR, IWR, and irrigation scheduling for major crops grown in the Hyderabad district of Pakistan using the CROPWAT model based on climatic, crop, and soil data.

Three types of data are required to use the CROPWAT software, namely climate data, soil data, and crop data. For this study, climate data were obtained from the websites of NASA, the FAO software of CLIMWAT (FAO 2018), and the World Weather Online. Climate data include maximum temperature, minimum temperature, relative humidity, wind speed, and sunshine hours. Table 1 shows the minimum and maximum temperatures in the study area.

Table 2 describes the air humidity, sunshine hours, wind speed, and precipitation in the study area. The study area is generally not characterized by high humidity. The maximum average humidity is 45–55% and the minimum average humidity is 29–35%. The precipitation pattern over the study area shows maximum precipitation during the dry summer period, June to September, with low average precipitation of 100–130 mm.

The reference evapotranspiration is the rate of evapotranspiration (ET) from a hypothetical crop with a height, albedo, and fixed canopy resistance, respectively (Allen et al. 1998). Whereas the fraction of rainfall that is stored in the soil profile and helps in the growth of crops is effective rainfall (P_eff).

The proper amount of water for irrigation and the proper timing of irrigation are determined by irrigation scheduling. Different agronomic techniques and irrigation scheduling under diverse geographical and climatic conditions have a big impact on getting the optimum yield (Mehrabi & Sepaskhah 2018; Ahmadi et al. 2022; Solgi et al. 2022). Irrigation scheduling under different management conditions and water supply schedules is developed after calculating ET_o, CWR, IWR using the CROPWAT model (Allen et al. 2005). The data entered into the CROPWAT and CLIMWAT software, such as crop type, cultivation date, and soil type medium (clay), included the meteorological station of the country Pakistan in the Hyderabad district. Once all the data were entered into the software, it calculated the CWR, IWR, and irrigation scheduling for the major crops grown in the Hyderabad district.

The total CWR, effective rainfall, and IWR for different crops in various agro-ecological zones, obtained after the application of respective data of the study area in the CROPWAT model, are described in Tables 4–7. The CWR, effective rainfall, and IWR for studied crops such as wheat, cotton, and banana, for the entire crop season are summarized in Table 8. The total CWR for the entire growing season for sugarcane, banana, cotton, and wheat were found to be 3,127.0; 2,012.3; 1,073.5; and 418.9 mm, respectively. However, the IWR for sugarcane, banana, cotton, and wheat for the entire growing season was found to be 2,964.0; 1,966.7; 1,052.7; and 407.6 mm, respectively. Settia et al. (2022) reported that rainfall is a major input for various water management and hydrological studies. However, the proportion of rainfall (effective rainfall) was 163.0; 45.6; 20.8; and 11.3 mm during the growing season of sugarcane, banana, cotton, and wheat, respectively. The results showed that the water requirement of all the selected crops in the study area was higher in the dry season than in the wet season, indicating that the crops grown in the dry season require more water than those grown in the wet season and also absorb a large amount of water due to the hot climate of the study area (FAO 2019). This is consistent with the FAO (2019) report, which states that crops grown in the rainy season require less water than those grown in the development and growth phases and that crops in the growth phase also require a large amount of water due to the high reference value of evapotranspiration in the months of the growth phase and require the more water as compared to the other three phases.

After analyzing the results obtained with the CROPWAT model, it was found that the crops with a longer growing period such as sugarcane and bananas, which occupy almost all the months of the year, consume abundant water. On the other hand, crops with a shorter growing season required less water for irrigation. The increased irrigation demand during the dry months can be explained by the drought and the resulting low relative humidity due to the lack of rain combined with high temperatures that led to increased evapotranspiration. In addition, the water demand of agriculture is highest during the hottest period with the highest temperatures, resulting in high evapotranspiration and a decrease in soil moisture (Zhong et al. 2014). Throughout the Hyderabad district, water losses from irrigation systems are significant, as water is generally transported to farmers’ fields through very poorly maintained earthen distribution systems, which experience significant water losses.

The results of a study conducted by Khan et al. (2021) to determine the crop water requirements for wheat and cotton in Sudan are comparable to those for wheat and cotton in the current study. The comparability of the outcomes supports our conclusion. Additionally, the predicted IWR for wheat in this study is comparable to recent studies done for Peshawar by Khan et al. (2019). Additionally, the results of the CWR, IWR for wheat, cotton, banana, and sugarcane of the Nawabshah district of Pakistan reported by Solangi et al. (2022) are consistent with recent research.

Irrigation scheduling is a simple tool to determine how much water to deliver to crops and when. Each crop has several stages, namely the initial stage, the developmental stage, the middle stage, and the late stage. At each stage, the irrigation requirement is different, so irrigation must be properly planned for the optimal use of water (Solangi et al. 2022).

In this study, it was found that the irrigation requirement for each crop was lower in the early stage and then increased in the developmental stage. In addition, it was approximately constant, and, with the exception of bananas, it was highest in the middle phase, while it decreased in the late phase due to the fact that the land must be dry to facilitate harvesting.

Table 9 obtained by the CROPWAT model, describes the net irrigation for the wheat crop as 300.7 mm and the gross irrigation as 429.6 mm. While Table 10 shows the net irrigation for the cotton crop as 828.1 mm and the gross irrigation as 1,284.8 mm. However, Table 11 obtained by CROPWAT model, describes the net irrigation and gross irrigation for the sugarcane crop as 1,640.2 and 3,371.6 mm, respectively. Table 12 obtained by CROPWAT model, describes the net irrigation for the banana crop as 1,640.2 mm and the gross irrigation as 2,225.7 mm.

The analysis revealed that the total CWR for the entire growing season for sugarcane, banana, cotton, and wheat were 3,127.0; 2,012.3; 1,073.5; and 418.9 mm, respectively. However, the IWR for sugarcane, banana, cotton, and wheat for the entire growing season was found to be 2,964.0; 1,966.7; 1,052.7; and 407.6 mm, respectively. Whereas the proportion of effective rainfall was 163.0; 45.6; 20.8; and 11.3 mm during the growing season of sugarcane, banana, cotton, and wheat, respectively. For the crops that include a dry season in their life cycle, all except wheat were found to be high with an IWR of 401.7 mm/season due to some parameters affecting the reference evapotranspiration (ET_o). In addition, an increase in the IWR value was observed for crops whose life cycle spans more, such as sugarcane with an IWR value of 3,094.5 mm/season. It was also found that rainfall significantly reduced the amount of IWR. During the dry season, CWR and IWR were higher due to high temperatures and low relative humidity, resulting in an increase in evapotranspiration. As for irrigation scheduling, the irrigation requirement for each crop was lower in the initial stage and then increased in the development stage. Moreover, it was approximately constant, and, with the exception of banana, it was highest in the middle phase, while it decreased in the late phase to facilitate harvesting. The study recommends the use of scientific tools such as CROPWAT and CLIMWAT to evaluate the CWR, IWR, and irrigation scheduling with a high degree of accuracy that farmers around the world, including Pakistan, readily accept. The present study will help to improve the monitoring of water resources and production. It would also help farmers, policymakers, and local farmers to make the best use of the extremely scarce resource of the country. In addition, engineers and planning agencies can use this study to design irrigation systems that are precisely matched to the water needed to be delivered to the fields.

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

The authors declare there is no conflict.