Irrigation Tools and Strategies to Conserve Water and Ensure a Balance of Sustainability and Proﬁtability

Efficient management and conservation practices for agricultural water use are essential for adapting to and mitigating the impacts of the current and future discrepancy between water supplies and water demands [...]


Introduction
Efficient management and conservation practices for agricultural water use are essential for adapting to and mitigating the impacts of the current and future discrepancy between water supplies and water demands. The importance of water conservation in agriculture is fundamental to ensuring a balance of sustainability and profitability. While profitability is a primary concern in any sustainable enterprise, farmers typically adopt new tools and practices that result in higher profits or reduced risks. Irrigation management practices that reduce water use with acceptable impacts on production would be viable strategies and cost-effective tools to cope with diminished water supplies and generate new sources of water to transfer for other agricultural uses and urban and environmental demands.
The water conservation measures may reduce crop water use and/or improve efficiency, and consist of advanced irrigation scheduling, deficit irrigation, on-farm irrigation system conversion and improvement, tailwater recovery systems, precision irrigation, and crop rotation and alternative low water use cropping systems. This Special Issue focuses on "Agricultural Water Conservation: Tools, Strategies, and Practices", which aims to bring together a collection of recent cutting-edge research and advancements in agricultural water conservation.

Deficit Irrigation Strategies
Several studies investigated the effect of mid-summer irrigation cut-off (no irrigation after June until the following spring) on alfalfa yield [1][2][3]. Moderate deficit irrigation strategies applying 12.5-33% less irrigation water than farmers' normal irrigation practices during the summer period were evaluated in the low desert of California [4]. This strategy demonstrated a promising and decent amount of water conservation and simultaneously generated desirable hay yields and quality. Implementation of the proposed summer deficit irrigation strategies on alfalfa could provide a reliable source of seasonally available water as well as sustain the economic viability of agriculture in the region. These strategies might be sustainable as an effective water conservation tool if such measures provide adequate economic incentives to the participating farmers. Incentive programs to farmers must offset the risk of implementing the proposed practices, as a tool for adopting water conservation practices.
The effect of drip irrigation integrated with partial root drying (PRD) and soil mulching was studied on squash plants [5]. The PRD strategy improved both the squash yield and water use efficiency (WUE). Soil mulching enhanced the physiological properties of the squash plants, fruit quality, squash yield, and WUE. This study reported that sowing squash plants in the winter season, applying water to compensate 50% of evapotranspiration, and using plastic mulch as water-saving strategies could be used as a water-saving strategy without reducing yields.

Water-Conserving Irrigation Practices in Row-Crops
A survey conducted among row-crop farmers in Mississippi [6] showed that the amount of irrigated area, years of education, perception of a groundwater problem, and participation in conservation programs are positively associated with practice adoption, while the number of years farming, growing rice, and pumping cost are negatively associated with adoption. However, not all factors are statistically significant for all water conservation practices. Survey results indicated that only a third of growers are aware of groundwater problems at the farm or state level; this lack of awareness is related to whether farmers noticed a change in the depth to water distance in their irrigation wells. The study recommended adopting more efficient irrigation technology and practices and precision agriculture technologies, such as soil moisture sensors and irrigation automation.

Accurate Estimates of Evapotranspiration for Irrigation Scheduling
Various methods of evapotranspiration measurement have been discussed by several researchers [7][8][9]. The most common methods for evapotranspiration (ET) measurement may be categorized as hydrological approaches (soil water balances and lysimeter measurements), micrometeorological approaches (eddy covariance, surface renewal, and Bowen ratio energy balances), and plant physiology approaches (chamber systems and sap flow measurements) [7]. In recent decades, satellite-based ET estimates using vegetation indices and scintillometer systems were developed as a result of rapid advances in instrumentation, data acquisition, and remote data access. The surface renewal method may estimate the surface fluxes at a relatively low cost, ultimately improving calculations of evapotranspiration and providing an economical tool for improving crop water management [10].
Remote sensing-based ET estimation is considered a promising tool for irrigation water management; however, uncertainties associated with satellite-based ET estimation still exist, especially with various remotely sensed platforms due to variations in spatial and temporal resolution [11]. In a study, satellite-based ET was evaluated using Landsat under semi-arid conditions in Texas under irrigated and dryland conditions. The Landsat-based ET overestimated the measured ET early and late in the growing season and underestimated ET during the peak of the growing season. More satellite-based ET assessment under arid and semi-arid conditions is required, where the magnitude and frequency of precipitation are erratic, and irrigation is the only source under arid conditions to replenish crop water needs [11].

Water Efficient Crop Management
A study was conducted to determine the optimum irrigation levels, row spacing, and tillage to maximize WUE while maintaining stable forage production [12] in pearl millet (a warm season C4 grass well adapted to semiarid climates) in the semi-arid region of the Southern Great Plains, USA. The greatest average forage production was achieved with the highest irrigation level; however, the greatest WUE was attained in tilled soil due to greater LAI, light interception, and plant growth than in no-till [12]. While the application of water increases the forage production, low LAI values increase E (soil evaporation) and reduce WUE, especially without adequate nutrient application.
The adoption of integrated management strategies will be useful for growing tolerant genotypes under saline water conditions and increasing water use efficiency. For the sustainable management of crop growth in saline environments, soil-crop-water management interventions consistent with site-specific conditions need to be adopted [13]. A study conducted in Tunisia [14] recommended that farmers with higher salinity water for irrigation should grow tolerant barley genotypes, allowing them to reduce the cost, on average by 30%. Changing cropping patterns is also regarded as a useful strategy for the rehabilitation and management of saline soils, especially when only saline water is available for irrigation [15,16].
Postharvest drip irrigation of asparagus cultivated in very light sandy soil significantly contributed to an increase in crop productivity. A significant increase in the height, number, and diameter of summer stalks, as well an increase in the marketable yield, weight, and number of green spears were observed for several American asparagus cultivars [17].
Ground-based remote sensing data of NDVI and canopy temperature along with soil moisture and ET-based smart controllers were assessed for efficient irrigation management of hybrid bermudagrass and tall fescue turfgrass in central California [18,19].

Irrigation with Wastewater
Irrigation with wastewater may contribute to the reduction of water abstraction in agriculture with an especial interest in arid and semiarid areas. The results of a study conducted in Italy [20] suggested that low-diluted hydrocyclone filtered digestate liquid fractions could directly be injected in a drip irrigation system with few drawbacks for the system. It may significantly contribute to water conservation since such wastewater are available from the late spring to the early fall when water requirements are high in the region [20].

Surface Irrigation Improvements
The results of a study conducted in Mexico showed that an efficient design and operation of surface irrigation considering initial irrigation tests and evaluation, characterization of irrigation plots, and calculation of the optimal flow rate using an analytical formula may considerably reduce irrigation-applied water [21]. The study demonstrated that irrigation application efficiencies increased more than 100% in some cases, while the WUE increased by 27, 38, and 47% for sorghum, barley, and corn, respectively [21].

Improvements in Small-Scale Irrigation Systems
Solar-powered drip irrigation using solar MajiPump along with conservation agriculture (CA) farming systems were found to be efficient to expand small-scale irrigation and improve productivity and livelihoods of smallholder farmers in Ethiopia [22]. Compared to the farmers' practices, water productivity was significantly improved under the CA farming and drip irrigation systems for both irrigated vegetables (garlic, onion, cabbage, potato) and rainfed maize production.