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. Key words infiltration; porous clay pipe; subsurface irrigation; wetting front; soil texture; hydraulic conductivity; matric potential; unsaturated flow; model simulation Simulation de l'infiltration à partir d'un tuyau d'argile poreux en irrigation souterraine Résumé Les améliorations apportées à la méthode traditionnelle d'irrigation souterraine par tuyaux d'argile poreux dépendent de la connaissance de la distribution de l'eau dans le sol. Il est important de connaître les relations qui existent entre les paramètres hydrauliques et physiques du système, à la fois pour la conception et la gestion du système. Pour simuler l'infiltration à partir du tuyau d'argile poreux et prédire la géométrie de la zone humide dans le sol, un modèle de simulation informatique a été développé. Des expériences en laboratoire ont été conduites sur des échantillons de sol présentant deux textures différentes dans un silo spécialement conçu pour comprendre le phénomène d'écoulement et pour alimenter la validation du modèle. Dans une texture de sol donnée, la profondeur d'installation du tuyau et le volume d'eau déversé dans le sol sont les principaux facteurs à affecter la zone humide. Les relations entre les différents paramètres que sont l'espacement latéral, la profondeur d'installation, la durée d'irrigation, la conductivité hydraulique du tuyau et la charge hydraulique dans le système, ont été établies en utilisant le modèle développé.
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 bene®cial 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 bene®cial. However, the issue of scaling should be also considered because the knowledge on the ®eld 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 ef-®ciency, 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 Agricultural Water Management 54 (2002) 205±225
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