Journal of Irrigation and Drainage Engineering

Rubber dams are constructed by the Agriculture and Fisheries Department in Hong Kong for the replacement of polluted agricultural weirs that are still in use. The main purpose of these dams is to impound water for irrigation. The reason the dams may be used as devices to mitigate flooding is that they are inflatable and deflatable. When a dam is inflated, it serves the function of an agricultural weir (low-level dam). However, when a dam is deflated, the barrier to water flow is removed, and hence the dam functions as a flood mitigation device. When the river water level reaches a preset level, the dams deflate automatically, and the risk of flooding is therefore reduced. Although rivers in Hong Kong are relatively small, they can become violent during severe rainstorms, resulting in flooding in many parts of the New Territories, north of the Hong Kong Island and the Kowloon Peninsula, from time to time. Due to the versatility of the rubber dams, they are of great benefit to Hong Kong's local farming communities.

This paper describes the detailed investigation on a 9 ha irrigated site with high water tables in the Tragowel Plains (Australia) of salt transport processes from shallow ground water to surface drainage. An intensive field monitoring program was established to record surface runoff and drain stage, fluctuations in water table levels, changes in the soil moisture profile, climatic data for estimation of evapotranspiration, changes in vegetation cover, rainfall and irrigation intensities, and field and laboratory measurements of a range of soil properties. The distributed, physically based European Hydrological System model was used to represent the processes using the field characteristics obtained for input parameters as well as time series flow and level data for calibration. Despite several limitations of the model, the significant physical processes occurring, and the range of their relative contributions of salt load to the surface drainage, were identified.

This paper describes the development of MIDASS, a lumped conceptual model for the prediction of streamflow and salt yield from irrigated catchments with high water tables. The model is suited for application to medium-sized catchments, though several models can be linked into a cascade arrangement for the simulation of catchments several hundred square kilometers in area. The model requires daily values of rainfall and irrigation deliveries as well as potential evaporation. As streamflow routing is not undertaken, model output is suited to weekly or monthly estimates of yield. The model was calibrated on a gauged 465 km² subcatchment of the Tragowel Plains Irrigation Area in south eastern Australia. Considering differences in scale, the relative proportions of baseflow, exfiltration, and runoff were found to agree reasonably well with physically based model results from a 9 ha study site. The model was applied to the Tragowel Plains to test the impacts of a number of irrigation and drainage management options on downstream river salinity.

A framework for determining the effect of maintenance events on irrigation system flows is described. Standard definitions for corrective and preventive maintenance are presented and two maintenance objectives and six classifications are established. Maintenance activities and decision criteria common to many irrigation systems are suggested. A format for describing these and other maintenance activities is proposed. A methodology for setting decision levels for maintenance activities is presented. Maintenance cost is compared with income lost as a result of less than maximum production because water supplied was insufficient for crop requirements. This comparison demonstrates that maintenance decision levels should be set so that maximum evapotranspiration can be achieved. Budget request forms and report forms are presented with examples of actual maintenance events showing the expected and actual impact on system flows.

A software package called Command Area Decision Support Model (CADSM) was developed to estimate aggregate crop-water requirements and to study management options for irrigated areas. Daily water and salt balances are simulated for individual fields within command areas based on crop type and stage of development, field characteristics, soil properties, possible ground-water contribution, salinity level, and several queuing factors that take cultural practices into account. The water requirements for individual fields are aggregated to estimate the potential command area water requirement, then totaled to determine the potential irrigation system demand. A queuing system is used to allocate available water to command areas and fields. Average crop yield response is predicted considering root-zone water deficit, salinity concentration, and water logging. Daily weather data, including evaporation, air temperature, and reference crop evapotranspiration (ET_o) can be generated using monthly statistical means and standard deviations. Model verification and calibration studies were conducted using various climatological data, cropping patterns, and simulated field conditions. Model results are shown to correlate well with measured field data from Utah and Thailand.

Hydrograph separation techniques are used to interpret metals (Fe, Cr, Cu, and metalloid As) data within the Aberjona River watershed, a highly urbanized watershed located near Boston. Results indicate that streamflow can be separated into three separate components: quick storm flow, slow storm flow, and long-term baseflow. Quick storm flows presumably activate highly mobile sources of sediment from outside the river and result in large increases in concentrations during storm events. Slow storm flows and long-term baseflows are characterized by low and relatively constant suspended-sediment concentrations. Variability of metal transport can be explained by assigning a dissolved-metal concentration to each streamflow component and a particulate-metal concentration to each suspended-sediment component. The net concentration of dissolved and particulate metals observed in the river is a result of mixing the components. Differences between subbasins can be addressed by postulating that the concentration of metals associated with streamflow and suspended-sediment components from each subbasin were different.

This paper describes a simple, few-parameters simulation model for a water harvesting-strip farming system. The model includes runoff threshold functions for different soil types, and for different initial soil water contents. The evapotranspiration component of the model accounts for dynamic root water extraction using regression equations that correlate canopy cover with root growth as a function of time. The model is simple to operate and uses readily available inputs. It can work with several crops when leaf area indices and crop stage coefficients are provided. The agreement between simulation outputs and field-observed data indicates that the model accurately describes the water balance within the system. The model is then recommended for obtaining preliminary design of water harvesting systems. A FORTRAN77 source code has been written for the simulation procedure.

A study was conducted to compare three existing solution methods for the inverse problem of unsteady open-channel flow (i.e., gate stroking). Both the accuracy of the results and the robustness of the methods were examined. Similar solutions were computed with a method-of-characteristics and an implicit finite-difference model, except under extreme transients, in which case the method-of-characteristics model performed better. However, the characteristic model was unable to find a solution for some proposed examples. An explicit finite-difference model was also examined and, although it produced satisfactory results for some examples, it is inherently unstable. A nonlinear, implicit finite-difference gate-stroking method was developed, which was sufficiently accurate and more robust than the existing methods.

The Sector Venturi is a novel design to determine discharge in sewers. Its main features are simplicity, accuracy of ±5%, and low cost. Further, the modular limit is comparable to standard flumes, and the capacity is equal to similar devices. Based on an optimization procedure, the best geometry was retained. The Sector Venturi is characterized by a ring of variable diameter, in which two sharp-crested sectors are attached. When mounting the Sector Venturi, the ring is expanded to match with the pipe section. The sector elements are sharp crested and have a notch at the invert. The principle of discharge measurement involves critical flow across a constriction. To stabilize the depth reading, holes are drilled in the sectors and two symmetrically arranged containers allow reading of the total head. The discharge is determined with a rating curve. One particular feature of the Sector Venturi is its insensitivity to clogging. As outlined, there are several disadvantages when compared with the pier-shaped Mobile Venturi Flume, but these do not detract from a useful means of discharge measurements in sewage.

The effect of soil salinity and soil-water content on the apparent soil electrical conductivity as measured with the EM-38 electromagnetic conductivity meter has been investigated. Results showed substantial changes in the meter's readings as soil-water content changed. The higher the soil salinity, the more sensitive the meter's readings were to changes in soil-water content. A linear relationship existed between soil-water content and apparent electrical conductivity for each level of soil salinity over the range of measured soil-water contents.