Special Issue on Trends and Challenges of Sustainable Irrigated Agriculture

In the twenty-first century, the shortage of freshwater is one of the most important environmental concerns facing several regions of the world because of the growing demand of increasing population, agricultural intensification, and economic growth. Global climate change will contribute to exacerbate the problem, generating new drought-prone areas and increasing those already characterized by severe aridity.

Worldwide it is estimated that, on average, agriculture accounts for 70% of the total water consumption, compared with 10% for domestic consume and the remaining used by industry. Moreover, according to FAO estimates, by 2050 agricultural production has to increase by 60% to satisfy the demands for food and feed (FAO 2013). Within this context, it is necessary to think back and make effective policies and actions for enhancing rational land use planning and agricultural inputs for a better exploitation of the existing technologies, even to rise the farmers’ awareness on the consequences of water scarcity.

Sustainable agriculture must therefore be prescribed as a policy approach to maximize production while maintaining environmental quality in a fragile and quite stressed environment (Provenzano et al. 2013; Cammalleri et al. 2013b). It requires the conversion of current agricultural practices toward systems more productive and resilient to climate variability, in which land, water, and other inputs would be more efficiently used, and crops yield would be less variable. Heading forward to achieve these goals, short- and long-term strategies across different and integrated pathways are required. These would have to keep in mind issues such as food security and agricultural development, and take into account the existing environmental constrains.

According to FAO (2013), climate-smart agriculture (CSA) would be an integrated approach to achieve the goals of a sustainable development. It addresses the food security and climate challenges issues within the economic, social, and environmental dimensions of sustainable development.

Considering that in irrigated agriculture, water resources (both quantity and quality) are one of the major environmental constrains, which will intensify in the future, there is a priority for water management agents and stakeholders to consider its use sustainable. Thus, it is required and no longer postponed to improve technologies and approaches to optimize water use at different scales (farm, field, district, and higher).

On the one hand, it is necessary to increase the performance of irrigation systems, and on the other hand, it is crucial to adopt technologies for irrigation scheduling aimed to increase water use efficiency, avoiding wastes and losses.

This special issue reports the results of some research presented in the session 11.3. Soil and Irrigation Sustainability Practices, at the European Geoscience Union general assembly, held in Vienna in 2012, related, as summarized below, to applications of on-farm control sensors aimed to increase effectiveness of irrigation, to methodologies for improving agrohydrological models predictions, as well as to practices of good management in different crop-systems. The contributions of the papers in this volume are not intended to get into an exhaustive detail of the above issues; nevertheless, they highlight some advice to implement actions toward a more sustainable management of irrigated agriculture.

Playan et al. (2014) describe the importance of investment in water-efficient technologies aiming at maximizing economic return in on-farm irrigation systems. Then, present a list of existing opportunities that, under particular conditions, allows to maximize irrigation efficiency and water productivity overpassing the unsatisfactory water use efficiency, often recognized at farm level. In particular, agrohydrological models and soil sensors have been successfully used to control landscape irrigation, whereas intelligent and autonomous systems are effectively applied to monitor the climate and to drive the complex water and nutrient application in greenhouses. In addition, the development of affordable systems for drip irrigated orchards, for irrigation machines, and for solid-sets sprinkler irrigation still remain in the domain of science and technology. In fact, according to the authors, in drip irrigated orchards with automated deficit irrigation, the need of a high number of sensors for the continuous and precise monitoring of soil and crop water status, as well as the required skills, often limit this methodology. Moreover, for self-propelled sprinkler irrigation machines, the main concern still remains in the difficulty of set input application to field variability despite the progress in automation and also in the possibility of adaption under different soil-crop systems. The paper finally presents an on-farm controller device driven by simulation models that can be used for a solid set sprinkler system, in order to reduce the effects of meteorological conditions on wind drift and evaporation losses. The opportunities and the limitation of their different alternatives, according to site-specific conditions, are illustrated and discussed.

A methodology to derive standardized reference evapotranspiration zone maps by daily climate data and GIS is proposed in the paper by Mancosu et al. (2014) that describes its application for Sardinia region (Italy). The characterization of irrigated zones by class of reference evapotranspiration open the window to evaluate crop water requirements on large areas and/or to investigate the impact of climate change. Reference evapotranspiration was estimated by means of the UN-FAO Penman Monteith equation (PM), as later modified by ASCE-EWRI, for the meteorological stations providing the full required dataset (solar radiation, air temperature, wind speed, relative humidity). Likewise, for meteorological stations providing only air temperature (partial data), it is shown that the PM equation generally provides better estimations of reference evapotranspiration than the Hargreaves-Samani equation, even after the necessary calibration taking into account site-specific climate conditions, with solar radiation, wind speed, and humidity data collected by nearby full stations. In addition, reference evapotranspiration zone maps were then derived by the ordinary kriging model applied to the data collected in 63 meteorological stations. Their results fitted the observed data better than the radial basis function or the inverse distance weighting method.

An amendment of FAO-56 dual approach model is proposed in the paper of Rallo et al. (2014), with the aim to improve the estimations of actual transpiration fluxes of drought-tolerant olive trees, maintained under soil water deficit conditions. Importance of a correct estimation of daily evapotranspiration fluxes in Mediterranean environment is considered a key step to challenge the increasing reduction of water availability (Cammalleri et al. 2013a). The proposed modified version of the model considers a more realistic convex shape of the water stress function, as experimentally obtained for the examined crop (Rallo and Provenzano 2013), to substitute the original linear function and assumes the minimum soil water content measured in the field, instead of the frequently used wilting point. The amended model was then validated by comparing simulated soil water contents and actual crop transpiration fluxes with the corresponding measured in the field during three irrigation seasons. Moreover, its aptitude to simulate crop water stress coefficients has been assessed on the basis of independent field measurements of midday stem water potentials. The modified version allows a better modelling of the root water uptake than the original. Therefore, it not only enhances the estimation of actual transpiration fluxes, as a consequence of the better schematization of the stress function, but it improves the evaluation of the thresholds defining actual soil water status. Anyway, for the investigated crop, further improvements of the model are evoked to quantify the contribution of tree capacitance on transpiration fluxes, as well as to define the seasonal dynamic of the actual soil volume where root water uptake occurs, responsible of some local discrepancies observed between measured and simulated actual transpiration fluxes.

The need of systematic investigations aimed to derive reliable water status thresholds to control deficit irrigation is emphasized in the paper of Kloss et al. (2014), who propose a methodology to assess the performance of a new sensor for monitoring soil matric potentials in a wide range (from $\mathrm{pF}=0$ to $\mathrm{pF}=7$), as well as to define thresholds of soil water tension in which the crop yield reductions are absent or limited. First, an experimental test was carried out in a greenhouse where maize plants, growing in containers, were maintained under deficit irrigation in order: (1) to test the performance of the new sensor, and (2) to calibrate the crop growth model Daisy (Abrahamsen and Hansen 2000). Then, the data were used to define the water status thresholds. The sensor can be considered a promising tool for sensor-based drip irrigation scheduling under deficit conditions because it can measure a wide range of water tensions. Moreover, according to model simulations, a threshold of pF slightly lower than 3.5 should maintain the highest productivity under different irrigation scheduling strategies. However, further research is necessary in order to verify the representativeness of the approach and to extend the validity of the results to field conditions.

The paper of Pascual-Seva et al. (2014) describes the effect of planting strategies and irrigation systems on yield and water use efficiency of chufa, a typical crop in Valencia area (Spain). In particular, three planting strategies, considering ridges with a plant row and flat raised beds with two or three plant rows, under furrow and drip irrigation were studied for two years. According to the continuous measurements of soil water contents, monitored with capacitance sensors, irrigation water was provided when in the ridge with a plant row soil water content dropped to 60 or 80% of the field capacity, respectively for furrow and drip irrigation. Experiments evidenced that planting in flat raised beds, and particularly with two plant rows, increases crop yield without diminishing crop tuber size. Moreover, the crop yield and, therefore water use efficiency drip irrigation systems, were significantly higher in drip irrigation systems than in surface irrigation despite of the lower water application in the former.

The case studio presented by Célia de Matos Pires et al. (2014) discusses the effects of subsurface drip irrigation and plant spacing on stem yield and on technological quality of sugarcane, as experimentally determined during four cultivation crop cycles. Experiments considered three different spacing between plant rows, with and without irrigation. In irrigated plots, water and fertilizer were provided with a subsurface drip system, whereas for rainfed cultivation fertilizers were manually applied to the plant rows. The results showed that in general, the sugarcane’s yield was not influenced by irrigation except during the second ratoon, when the quality of the juice and the values of total recoverable sugar resulted higher than those obtained in the rainfed plots, thus demonstrating the benefits of subsurface drip irrigation to sugar cane properties.

Finally, the paper of Gabriel et al. (2014) analyzes the impact of cover crop, practiced with barley (Hordeum vulgare L.), vetch (Vicia villosa L.), and fallow treatment, sown during the intercropping season, on water, nitrogen, and salinity dynamics of a maize (Zea mays L.) cropping system. The results, obtained on the basis of field experiments and simulation models, show that in irrigated systems, the replacement of fallow with cover crop can be effective in reducing nitrate leaching, without increasing soil salinity or, even better, reducing the top layer salinity at maize sowing. Moreover, the reduction of net salt loss observed in the plots with cover crop, compared with fallow, allows to limit irrigation volumes for salt leaching, as well as reducing the risks of deep water contamination by nitrates.