Rainer Loof

Application of GIS and crop growth models in estimating water productivity

Abstract Tighter competition in water use is projected in the future. As water demand increases, water related problems could happen along the way. Accordingly, issues on water availability and use could be crucial to study to search for ways and means on how to cope up with the present trend. Sound water management practices could play a key role to the solution of problems relating to water availability and use. Water use in agriculture is considered the highest among other water users because of the water intensive processes involved in it. Aside from the crop water requirements, water loss, which are not beneficial to crop processes can add a huge volume to the total water usage in agriculture. Base from this argument, there could be greater possibility to save water from agriculture, which can be used for other purposes thereafter. To explore this option, analysis at the crop level could be beneficial. However, the issue of scaling should be also considered because the knowledge on the field scale could not be generally true in the basin scale. The objective of the study was to apply crop growth simulation models coupled with geographic information system (GIS) to analyze water productivity, which is an indicator of water use efficiency, at the basin scale. The methodology was applied to Laoag River Basin in Ilocos Norte, Philippines to study water productivity in spatial and temporal dimensions. Three crops were considered in the analysis: rice, maize and peanut. Simulations were done for both existing and potential agricultural areas. The potential productions of the selected crops from October 1996–September 1997 were used as bases in determining water productivity for the three cropping seasons (CS) being considered in the study. Water-limited productions were simulated for each of the crops, for each of the CS in the basin. Moreover, a marginal productivity analysis was done to determine the potential of water for crop production in the basin. Subsequently, the significance of irrigation was emphasized in the analysis when availability of water, and the combination of water and nitrogen (N) are limiting, respectively. The results showed that the spatio-temporal analysis of water productivity could provide substantial information for water saving opportunities and, hence, strategies in irrigated agriculture.

Performance based irrigation planning under water shortage

A linear programming (LP) based optimization model and a simulation model are developed and applied in a typical diversion type irrigation system for land and water allocation during the dry season. Optimum cropping patterns for different management strategies are obtained by the LP model for different irrigation efficiencies and water availability scenarios. The simulation model yields the risk-related irrigation system performance measures (i.e. reliability, resiliency and vulnerability) for the management policies defined by the optimization model. The alternative strategies are evaluated in terms of all performance criteria (i.e. net economic benefit, equity and reliability) simultaneously through a trade-off analysis using a multi-criteria decision making method (compromise programming). For the case study of the Kankai irrigation system in Nepal, with equal preference to the objectives, a management strategy with equal share of water among the project subareas appears to be the most satisfactory alternative under water shortage conditions. The existing water allocation policy is not economically efficient. Deficit irrigation in Early paddy appears attractive under favorable hydrologic scenario, particularly if accompanied by measures to improve existing irrigation system efficiency.

Simulation of infiltration from porous clay pipe in subsurface irrigation

Abstract Advances in the traditional method of subsurface porous clay pipe irrigation rely on knowledge of the distribution of water in the soil. Knowing the relationships among the hydraulic and physical parameters in the system is important for both the design and management of the system. To simulate the infiltration from the porous clay pipe and predict the wetted zone geometry in the soil, a computer model is developed herein. Laboratory experiments were conducted on soil samples representing two different soil textures in a specially designed bin to understand the flow phenomenon and to validate the developed model. In a given soil texture, the installation depth of the pipe and the volume of water applied in the soil are the major factors affecting the wetted zone. The relationships among various parameters, namely lateral spacing, installation depth, irrigation run time, hydraulic conductivity of the body of the pipe, and hydraulic head in the system, were established using the developed model.

Fingered preferential flow in unsaturated homogeneous coarse sands

The occurrence of fingered preferential flow depends on both flow and porous media characteristics. The boundary condition of unsaturated infiltration has been investigated to assess whether it produces unstable wetting in homogeneous coarse sands. Laboratory tracer experiments were conducted in two coarse sand materials; for each sand material two initial conditions of air dry and field capacity were considered. Results indicate that the commonly occurring process of nonponding rainfall can provoke fingered preferential flow in homogeneous sandy soils. This phenomenon is very predominant in dry as compared to wet initial conditions. Models of water flow and solute transport in uniform coarse soils should incorporate the effects of such fingered flow phenomenon.

Equity aspects of irrigation development: Evidence from two systems in the hills of Nepal

Abstract Equity is of great relevance to developing irrigation. Its two dimensions —horizontal in regard to water distribution to farmland and vertical in terms of productivity differences between farm categories — are studied in two irrigation systems, an old farmer‐managed and a new government‐agency‐managed system. Equity in the distribution of irrigation water differed between abundant and scarce water supply conditions. Paddy grown during the monsoon season when water is available in abundant quantity shows that there was a reasonable degree of fairness in its distribution between head and tail reach farmland. However, wheat grown in the dry season with limited supply of water rendered evidence of unfair distribution demanding better management of irrigation water. The analysis of vertical equity shows that small farms are more efficient than large ones in increasing productivity through the use of irrigation facilities.

Simulation of stable and unstable flows in unsaturated homogeneous coarse sand

Abstract A numerical model developed to simulate stable and unstable flows in unsaturated porous media is described. Results of numerical studies carried out to simulate laboratory experiments with the assumption of stable flow demonstrate the occurrence of unstable flow for the initial conditions of both air dry and field capacity for unsaturated infiltration in sands. This indicates that the Richards flow equation based on moisture content and potential variables averaged over total crosssectional area may not be applicable for flow under instability-prone boundary conditions. The unstable flow due to wetting front instability is modelled using the steady-state theory proposed by Hillel & Baker (1988). Simulation results for fingered flux calculated with the theory represent the experimental data reasonably well. The pore water velocity remains constant irrespective of the incident flux as long as the flux is smaller than the hydraulic conductivity value at the water entry suction of the porous media. T...

Operational Plans and Performance in the Phitsanulok Irrigation System, Thailand

Key performance aspects of a 92 000 ha rice irrigation system in central Thailand are reviewed, with reference to water-distribution performance, agricultural performance and plan-implementation performance. Farmers have invested heavily in recent years in pumps, principally for access to shallow groundwater. This allows independent agricultural decision making by farm households. It is shown that tail-end farmers, who have the highest intensity of pumps, achieve the best agricultural performance. The performance in terms of canal water distribution and plan implementation is erratic. Absence of appropriate water-measurement facilities, insufficient information flows concerning water quantities and wide variations of farmer strategies seem to be significant reasons for these difficulties.

Improved operation of large irrigation canal systems in southeast Asia

Abstract Poor performance of the irrigation systems in Southeast Asian countries is a cause for concern considering the increasing water scarcity. Lack of adequate knowledge and systematic ignorance of main system management has been identified as one of the primary factors for the resulting poor performance of these systems. In this study a model has been developed for operating the main canal of these systems by integrating hydraulics of the flow and decision‐making technique. The model developed was applied to the main canal of the Phitsanulok Irrigation Project in Northern Thailand. The results of this application approved the concept and envisaged substantial improvement in the operation of the main canal. The model can in general be applied to similar irrigation systems in the developing countries.

Rainfall network design using the kriging technique: A case study of Karnali river basin, Nepal

Abstract Spatial variation structure of monthly and annual mean precipitation in the Karnali river basin, Nepal has been analysed by using kriging. The physiographic and climatological features of the basin are presented in sufficient detail to better understand the spatial and temporal variation pattern of the precipitation within the basin. The concept of ‘regionalized variables’ and the theory of kriging have been introduced. Based on the spatial variability of the precipitation obtained by kriging, a methodology has been developed for selecting the best locations for a given number of rain gauges planned to be added in a network. It has been demonstrated that kriging can be of valuable use in identifying the optimal locations for a set of additional rain gauges using kriging standard deviation as an indicator. The results obtained from kriging are realistic to the extent that the stationarity assumptions are true. Quantification of error nevertheless makes it possible to select locations of rain gauge...