Journal of Irrigation and Drainage Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Floodplain storage should be carefully considered among the structural measures for active flood risk mitigation. Floodplain storage facilities assure an attenuation effect and consequently a significant reduction and delay of the flood peak by allowing temporary storage of the flood volume in an adequate upstream capacity. This paper addresses off-stream floodplain storage. In off-stream floodplain storage, the reservoir is located outside the river bed and equipped with an adequate intake weir, which is usually controlled by downstream waterworks and outlet orifices. The results of an experimental survey conducted in the laboratory of the Department of Civil, Architectural, and Environmental Engineering at the University of Naples Federico II are presented to analyze the hydraulic behavior of off-stream floodplain storage filled by a side weir that is controlled downstream. The experimental results, which were obtained by varying the geometric characteristics of the weir, were compared with those inferred from a one-dimensional numerical model, highlighting small deviations and providing useful information for the overall design of the storage. The obtained results primarily showed that an increase in the storage volume causes an attenuation of the inflow hydrograph, achieving lower outflows and a decrease of the attenuation ratio $\eta$, thus increasing the storage efficiency. Finally, experimental data were compared with the results of a previous survey on in-stream storage, highlighting a higher efficiency of the off-stream floodplain storage.

Moisture retention relationships for coarse, high-infiltration soils are difficult to empirically determine and estimate. Present day software models for stormwater management (SWM) that are used as sizing and performance prediction tools for filtration Low Impact Development-Stormwater Management (LID-SWM) systems typically assume that these systems function under saturated flow conditions. This directly impacts prediction of system drainage and hydrographs, as well as the estimates of physically-based water quality improvement. Yet real-time monitoring of these systems demonstrated that saturation of the filter media is rarely achieved. This article presents a framework for obtaining the moisture retention curves (MRC) and relative hydraulic conductivity $K_r(\theta )$ function for engineered filter media and other hydraulic control soils used in four LID-SWM systems: pervious pavement, sand filter, subsurface gravel wetland, and bioretention. These functions needed in routing water through the filter media with unsaturated flow functions are developed from easily measurable soil properties like porosity and particle size distribution, and can be integrated in current available stormwater design software. The framework consists of a sequence of physically based equations: Arya-Paris for the $\theta (\psi )$ function, Bower for gravel content adjustments along with an extension of the $\theta (\psi )$ function proposed in this article, and Mualem for the $K_r(\theta )$ function. This sequence is combined with the Van-Genuchten fitting equation for soils with irregular particle size distributions.

Microirrigation plants, if properly designed, allow for water use efficiency to be optimized and high values of emission uniformity to be obtained in the field. Disposing paired laterals, for which two distribution pipes extend in opposite directions from a common manifold, can contribute to reducing the initial investment cost that represents a limiting factor for small-scale farmers of developing countries where in the last decade, the diffusion of such irrigation systems has been increasing. The objective of this paper is to propose an analytical approach to evaluate the maximum lengths of paired drip laterals for any uniform ground slope, respecting the criteria to maintain emitter flow rates or the corresponding pressure heads within fixed ranges in order to achieve a relatively high field emission uniformity coefficient. The method is developed by considering the motion equations along uphill and downhill sides of the lateral and the hypothesis to neglect the variations of emitters’ flow rate along the lateral as well as the local losses due to emitters’ insertions. If for the uphill pipe, the minimum and the maximum pressure heads occur at the upstream end and at the manifold connection, respectively, on the downhill side, the minimum pressure head is located in a certain section of the lateral, depending on the geometric and hydraulic characteristics of the lateral, as well as on the slope of the field; a second relative maximum pressure head could also exist at the downstream end of the pipe. The proposed methodology allows in particular the number of emitters in uphill and downhill sides of the lateral to be determined separately, and therefore, once fixing emitter’s spacing, the length of the uphill and downhill laterals and the position of the manifold. Applications and validation of the proposed approach, considering different design parameters, are finally presented and discussed.

Urban stormwater runoff management systems are usually designed to meet performance standards based on historical climate data, which are assumed to be stationary. Based on the evidence from climate change impact studies, in the near future, stormwater management systems, within the built environment, may need to meet performance expectations under climatic conditions different from historical climate. Considering the impacts of climate change on rainfall intensities and stormwater runoff peak flow and volumes, and in turn the effectiveness of mitigation, practices for urban stormwater management are desirable. This paper presents the results of a climate change impact study on urban stormwater runoff in the Bronx River watershed, New York City. Considering the impacts of climate change on watershed runoff, the potential for low-impact development (LID) controls to mitigate the impacts was investigated. Stormwater runoff and LID controls were modeled using the U.S. EPA Storm Water Management Model version 5 (EPA SWMM5). The simulations were driven by historical precipitation modified to represent future projections using a change factor methodology based on precipitation from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Using the change factor method, historical precipitation was perturbed to obtain future data, based on three climate scenarios projecting maximum, mean, and minimum values for rainfall. Results of frequency analysis performed on the simulated peak flow rates, with different recurrence intervals, showed a noticeable increase in the frequency of occurrence of extreme storm events and their peak values, under future extreme climate conditions. An LID control scenario consisting of rainwater harvesting, porous pavement, and bioretention was designed and evaluated using the SWMM model. The results showed that, while average increase in historical annual runoff volume under climate change impacts was approximately 48%, the LID controls could provide an average reduction of 41% in annual runoff volume. Application of the LID controls also reduced peak flow rates by an average of 8 to 13%. LID implementation decreased watershed runoff corresponding to 2-year and 50-year return periods, by 28 and 14%, respectively. In conclusion, retrofits with LID controls may provide not only their inherent benefits (e.g., runoff volume and water quality), but also additional climate impact mitigation benefits for stormwater runoff.

Drought is a natural disaster that has always had a severe impact on agriculture, especially rain-fed agriculture. Many studies have indicated that different indices, for example the standardized precipitation index (SPI), do not show acceptable efficiency in quantifying agricultural drought. In the current study, an index was utilized that is the so-called standardized effective precipitation index (SEPI). The SEPI employs effective precipitation during the rain-fed wheat-growing season using a two-layer soil-water balance model. In order to assess the efficiency of SEPI in representing and capturing fluctuations of drought years, SEPI was compared with two SPI time series considering different inputs (annual precipitation and growing season precipitation) during 1981–2008 in seven agro-meteorological stations. The results showed that SEPI had better performance for monitoring variations of the normalized wheat yield time series than two SPI-based indices in different climates. For instance, the maximum compatibility (percentage of captured drought years by agricultural drought indices) between SEPI and normalized yield is related to the Sararoud station, in which the SEPI could capture all seven drought years, while two SPI time series could trap only three years out of seven years.

The feasibility of small and medium dams in arid environments is studied. A novel method for prioritization of dam commissioning is proposed. The method focuses on key hydrologic and economic factors that affect the investment decision-making process. Ranking for priorities is developed in a geographic information system (GIS)-aided environment. Evaluated parameters are based on extensive field data including topographic surveys and site visits, land cover and soils data, precipitation data, runoff data, demography, construction costs, and operation and maintenance costs. Reservoir yield, reliability, and annualized cost per satisfied demand are the three main criteria used in the methodology. Results of a case study on 12 proposed dams in Assir Region, Saudi Arabia, indicate that assessment of these criteria is crucial prior to investment decision-making in arid areas. More than three quarters of the small and medium dams proposed have been found to have a high cost of water productivity and very low reliability, despite a high yield. The developed method highlights the degree of the economic and hydrologic feasibility of small and medium dams in arid areas.

Pre- (1 year) and post- (2 years) passive hydration data were used to evaluate the effects on spatiotemporal storages at two pastoral wetlands in the Lake Okeechobee (LO) watershed within the Everglades basin, Florida, United States. To partially restore the predrainage wetland storage and hydroperiod, passive hydration was implemented by raising the discharge elevation using a culvert structure with removable boards at the end of the drainage ditch that drained the wetland/upland system. During the wet season (July–October) groundwater and total storages (surface water plus groundwater; rainfall normalized) in the wetland footprints increased 17 and 12% during the posthydration periods at wetland 1. At wetland 2, these increases were 39 and 12%, respectively. When analyzed at the subwatershed (wetland plus upland) scale, total storages (rainfall normalized) during wet season increased 12% in both wetlands in the posthydration periods. Annual surface storage (rainfall normalized) increased from 25 to $188{\mathrm{m}}^3$ for wetland 1, whereas at wetland 2, it increased from 0.2 to $6{\mathrm{m}}^3$ during posthydration periods (annual) at subwatershed level. Differences in rainfall, storage areas, topography, and wetland hydrology masked the hydration effects and resulted in inconsistent results across years and wetlands. At the LO watershed-scale, increased storage was only 14% of the desired storage. Long-term data are needed before implementation of passive hydration for achieving storage goals.