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Abstract: The delta of the Colorado River in Mexico supports about a million hectares of riparian, marsh and estuarine habitats of international importance. Some of these habitats depend on flows of fresh and brackish water from the U.S. and Mexico. Up to now, these flows were the incidental result of water management actions taken to provide water for agriculture and municipal use, protect against flooding, and dispose of saline agricultural return flows. This paper briefly describes the wetlands and documents recent bi-national efforts to provide environmental flows to the delta, codified in Minutes 306, 316 and 319 of the water treaty between the U.S. and Mexico for the utilization of Colorado River water. Providing water for environmental uses in this watershed will be a daunting task given the many competing uses for water and expected diminished flows due to climate change. The paper serves as an introduction to a special issue of Ecological Engineering, Wetlands of the Colorado River Delta, which contributes 17 new research articles to the science based on these diverse aquatic habitats. We hope these studies will be useful to those developing management strategies to preserve and enhance these habitats for the future. [Copyright &y& Elsevier]

In a companion paper, the agricultural drought economic risk assessment (ADERA) model was described. ADERA studies agricultural drought not only in terms of deficit soil water content and its frequency and severity (as in the traditional approach) but also in terms of net benefit reduction and vulnerability. The relationship between drought index, crop yield, and crop economic net benefit is modeled and three threshold levels are considered: the critical water content; the critical impact in terms of net benefit; and its critical return period (i.e., frequency) above which the soil-climate unit is considered unsuitable for the cultivation of a specific crop. These critical levels are used in long-term action planning as the triggers for different risk classes. The rapidity with which the critical impact is reached is considered indicative of the crop vulnerability, and real-time monitoring of the impact evolution (short-term actions) is also implemented. Initially ADERA was applied at only one point (soil-crop-climate unit), but because agricultural drought is by definition a wide area phenomenon, the model must be validated in other units and then used to evaluate the drought area extension (risk map). In this paper, generalization, demonstration, and guidance on the application of ADERA are given with reference to rainfed sunflower and sorghum crops in six soil-climate units in central Italy for which time series for yield (obtained in experimental plots where the water scarcity was the principal factor limiting the final yield) since 1978 is available. The application of ADERA made it possible to quantify the vulnerability class and the corresponding risk class for the single unit (long-term action planning). The analysis results confirm that typically for the selected crops the critical impact has a low risk and a low vulnerability. In fact sunflower and sorghum are rainfed crops widely cultivated in the area under investigation. Moreover, the results show that each risk class is present only within a specific cost range, in particular the upper and lower limits, and the size of each cost range varies according to the different units; the occurrence of a specific risk class and vulnerability level is highly correlated with the selected triggers, in particular with the critical net benefit (for an increment of which above the corresponding cost range, a given soil-climate unit can become unsuitable for the cultivation of a specific crop); the critical water content is a normal occurrence event for all the units analyzed, with the crop water stress onset date always around 40% of the growing period being a typical characteristic of the specific area, of the two crops and of the selected drought index. The practical usefulness of the impact monitoring procedure was stressed by comparing its progressive evolution in the cases of both a rainfed crop and of a rescue irrigation carried out at a selected and opportune date. In the latter case the impact was reduced below the critical level. [ABSTRACT FROM AUTHOR]

Monitoring soil salinization has been a difficult process in arid lands due their large spatial and temporal variability. Hyperspectral remote sensing has offered a potential for faster detection of salinization process but mostly from empirical approaches. In this paper, an integrated approach combining model inversion and empirical regressions has been proposed for soil salt content (SSC) estimation from hyperspectral information obtained from controlled laboratory experiments. All soil samples were artificially salinized using \Na2\SO4, NaCl, and \Na2 \CO3 (99% purity) salts to different levels and to different soil-moisture conditions, since soil moisture often jointly affects reflectance spectra with SSC. Hapke model was calibrated and validated for its simulation on soil reflectance and showed good agreements with measured data. The optimal values of single scattering albedo that was inversely retrieved from the Hapke model had good relationships with SSC at 2000–2200 nm for each treatment even under various soil-moisture conditions. Taking usage of these findings, the integrated approach obtained high accuracies on SSC estimations with R^2's of 0.90, 0.86, and 0.72 and slightly dropped R^2's of 0.89, 0.81, and 0.67 for NaCl-, \Na2\SO4-, and \Na2\CO3-type saline soils under respective dry and wet conditions. The R^2 decreased to 0.55 and 0.53 for dry and wet soils when salt types were ignored. The integrated approach provides a novel as well as an efficient way for SSC estimation from reflected spectra, and hence, we foresee its potential applications for large-scale SSC mapping from reflectance measurements. [ABSTRACT FROM PUBLISHER]

Climate change impact reports by the Intergovernmental Panel on Climate Change (IPCC) are well established throughout the world. By now many local, regional, and central authorities are aware of these impacts and they try to take necessary precautions according to their environmental circumstances. Preliminary decisions cannot be taken unless there is a model for future climate change impacts, especially for rainfall occurrence, amount, and frequency estimations. The main purpose is to combine the general circulation (climate) model (GCM) outputs with the local meteorology station records by a suitable downscaling procedure. In this paper, a statistical quadrangle downscaling model (QDM) is developed for rainfall estimations up to 2100. This model helps to downscale the nearby four scenario rainfall amounts at GCM grids to a desired point. Two main steps in the model structure are the use of a regional dependence function (RDF) for transferring the GCM output data from the four nearest grid points to the location. The second step is to adjust the downscaled scenario monthly rainfalls with the local rainfall records. The GCM monthly rainfall outputs are downloaded from the National Center for Atmospheric Research (NCAR) with special report emission scenario A2. The application of the methodology is presented for Riyadh City, Kingdom of Saudi Arabia (KSA). The RDF is obtained from the most reliable meteorology records at 27 locations that are irregularly scattered all over the country. Then it is used for scenario data transfer from the regular grid points. The adjustment is based on the average statistical properties of concurrent monthly rainfall amounts between 1985 and 1996. The trend component in the GCM remains intact. It has been observed that there is a major increasing trend in the monthly rainfalls after 2050. In the meantime, each decade starting from 2011 gives estimates that indicate steady rainfall increases up to 2050. The QDM indicates a 40% increase in the annual total rainfall amounts, with a 15% standard deviation increase. [ABSTRACT FROM AUTHOR]

Net radiation (Rn) is the main driving force of evapotranspiration (ET) and is a key input variable to the Penman-type combination and energy balance equations. However, Rn is not commonly measured. This paper analyzes the impact of 19 net radiation models that differ in model structure and intricacy on estimated grass and alfalfa-reference ET (ETo and ETr, respectively) and investigates how climate, season and cloud cover influence the impact of the Rn models on ETo and ETr. Datasets from two locations (Clay Center, Nebraska, subhumid; and Davis, California, a Mediterranean-type semiarid climate) were used. Rn values computed from the 19 models were used in the standardized ASCE-EWRI Penman-Monteith equation to estimate ETo and ETr on a daily time step. The influence of seasons on the estimation of Rn and on estimated ETo and ETr was investigated in winter (November-March) and summer (May-September) months. To analyze the influence of clouds on the impact of Rn models, relative shortwave radiation (Rrs) was used as a means to express the cloudiness of the days as: 0≤Rrs≤0.35 for completely cloudy days; 0.35

The paper proposes a new methodology that studies agricultural drought not only in terms of deficit soil water content, its frequency and severity, as in the traditional approach, but also in terms of net benefit. For this purpose the relationship between drought index, crop yield, and crop economic net benefit is modeled and three threshold levels are considered. The first is the critical water content, below which the crop faces water stress (termed as potential agricultural drought). The second is the critical net benefit, below which there is meager (e.g., zero) profit (termed as actual agricultural drought). The third is the critical return period (or frequency) of the actual agricultural drought, below which there is an agricultural aridity (almost steady phenomenon with high impact, high risk) and above which there is an agricultural drought (occasional phenomenon with high impact, low risk). To pinpoint the time of occurrence of the potential and the actual agricultural drought, real-time monitoring is conducted. The rapidity with which these critical conditions are reached is considered as indicative of the crop vulnerability to the corresponding phenomenon. The evolved methodology therefore is based on an objective definition of agricultural drought and is formulated accounting for its peculiar characteristics. The methodology is called the risk analysis and economic impact assessment of agricultural drought (ADERA). ADERA is applied to the rainfed sunflower crop in the Papiano (central Italy) experimental plots for which the 26-year time series for yield and the corresponding phenological, climatic, and economic data are available. The drought index-crop yield-crop economic net benefit relationship makes it possible to quantify the frequency (return period) of the actual agricultural drought and to show that for the given costs and crop sale price the actual agricultural drought risk for the crop is low. For the case study, the vulnerability of the unit to the agricultural drought is low, as the critical net benefit typically occurs late in the growing season. [ABSTRACT FROM AUTHOR]

This paper presents the area and water allocation model (AWAM), which incorporates deficit irrigation for optimizing the use of water for irrigation. This model was developed for surface irrigation schemes in semiarid regions under rotational water supply. It allocates the land area and water optimally to the different crops grown in different types of soils up to the tertiary level or allocation unit. The model has four phases. In the first phase, all the possible irrigation strategies are generated for each crop-soil-region combination. The second phase prepares the irrigation program for each strategy, taking into account the response of the crop to the water deficit. The third phase selects the optimal and efficient irrigation programs. In the fourth phase of the model, irrigation programs are modified by incorporating the conveyance and the distribution efficiencies. These irrigation programs are then used for allocating the land and water resources and preparing the water release schedule for the canal network. [ABSTRACT FROM AUTHOR]

In this work, applications of the coupled solid set sprinkler irrigation and crop model AdorSim introduced in the companion paper are presented. The sprinkler irrigation model is based on ballistic theory, while the crop model is based on CropWat. AdorSim was used to evaluate the effect of sprinkler spacing on seasonal irrigation water use (WU) and crop yield. The most relevant results were related to the characterization of advanced irrigation scheduling strategies. The differences in crop yield and WU derived from irrigating at different times of the day were estimated for two locations strongly differing in wind speed. Irrigation guidelines were established in these locations to relate gross water use and water stress induced yield reductions. Simulations were also applied to estimate adequate wind speed thresholds for irrigation operation. In the experimental conditions, thresholds of 2.0–2.5 m s-1 proved effective to control yield reductions and to minimize WU. [ABSTRACT FROM AUTHOR]

The study was conducted to evaluate the potential of different strains of plant growth-promoting rhizobacteria (PGPR) to reduce the effects of salinity stress on the medicinal hopbush plant. The bacterium factor was applied at five levels (non-inoculated, inoculated by Pseudomonas putida, Azospirillum lipoferum + Pseudomonas putida, Azotobacter chroococcum + Pseudomonas putida, and Azospirillum lipoferum + Azotobacter chroococcum + Pseudomonas putida), and the salinity stress at six levels: 0, 5, 10, 15, 20, and 50 dS m-1. The results revealed that Pseudomonas putida showed maximal germination percentage and rate at 20 dS m-1 (18.33% and 0.35 seed per day, respectively). The strongest effect among the treatments was obtained with the treatment combining the given 3 bacteria at 15 dS m-1 salinity stress. This treatment increased the root fresh and dry weights by 31% and 87.5%, respectively (compared to the control). Our results indicate that these bacteria applied on hopbush affected positively both its germination and root growth. The plant compatibility with the three bacteria was found good, and the treatments combining Pseudomonas putida with the other one or two bacteria discussed in this study can be applied in nurseries in order to restore and extend the area of hopbush forests and akin dry stands. [ABSTRACT FROM AUTHOR]