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Taiwan average annual rainfall is approximately 2500 mm. In particular, 80% of the rainfall occurs in summer, and most of the heavy rainfall is caused by typhoons. The situation is worsening as climate change results in uneven rainfall, both in spatial and temporal terms. Moreover, climate change has resulted the variations in the seasonal rainfall pattern of Taiwan, thereby aggravating the problem of drought and flooding. The irrigation water distribution system is mostly manually operated, which produces difficulty with regard to the accurate calculation of conveyance losses of channels and fields. Therefore, making agricultural water usage more efficient in the fields and increasing operational accuracy by using modern irrigation systems can ensure appropriate irrigation and sufficient yield during droughts. If agricultural water, which accounts for 70% of the nation's total water usage, can be allocated more precisely and efficiently, it can improve the efficacy of water resource allocation. In this study, a system dynamic model was used to establish an irrigation water management model for a companion and intercropping field in Central Taiwan. Rainfall and irrigation water were considered for the water supply, and the model simulated two scenarios by reducing 30% and 50% of the planned irrigation water in year 2015. Results indicated that the field storage in the end block of the study area was lower than the wilting point under the 50% reduced irrigation water scenario. The original irrigation plan can be reduced to be more efficient in water usage, and a 50% reduction of irrigation can be applied as a solution of water shortage when drought occurs. However, every block should be irrigated in rotation, by adjusting all water gates more frequently to ensure that the downstream blocks can receive the allocated water to get through the drought event. [ABSTRACT FROM AUTHOR]

This study evaluated parameters of soil erosion and optimization of micro watersheds by applying a semidistributed basin-scale Soil and Water Assessment Tool (SWAT) model in various small watersheds of the Chakwal and Attock districts of Pothwar, Pakistan. The model was calibrated and validated on a daily basis for a small catchment (Catchment-25) of the Dhrabi watershed without any soil conservation structures. Statistical measures (R² and EN-S) were used to evaluate model performance; the model performed satisfactorily well for both surface runoff and sediment yield estimations, with the R² and EN-S values both being greater than 0.75, during calibration (2009-2010) and validation (2011). The model was applied to various small watershed sites in the Chakwal and Attock districts after successful calibration and validation. Soil erosion estimation was performed at these sites having loose stone soil and water conservation structures and being under various slope gradient and vegetation cover conditions. The structures had significant effects, and the average sediment yield reduction engendered by the loose stone structures at the various sites varied from 54 to 98 %. The sediment yield and erosion reductions were also compared under conditions involving vegetation cover change. Agricultural land with winter wheat crops had a higher sediment yield level than did fallow land with crop residue, which facilitated sediment yield reduction along with the soil conservation structures. Analyzing various slope gradients revealed that all selected sites had a maximum slope area of less than 5 %; stone structures were installed at these sites to reduce sediment yield. Based on slope classification analysis, the model was upscaled for the whole districts of Chakwal and Attock. The results indicated that 60 % of Chakwal (4095 km²) and Attock (3918 km²) by area lies in a slope range of 0-4 %; this thus implies that considerable potential exists for implementing soil conservation measures by installing stone structures. Estimates revealed that minimum sediment yield reductions of 122,850 t year-1 in Chakwal District and 117,540 t year-1 in Attock District could be achieved by installing loose stone structures in 60 % of the agricultural areas of both districts having a slope of 0-4 %; these findings can serve as a reference for policymakers and planners. The overarching findings of this study show that the SWAT model provides reliable results for sediment yield and soil erosion estimation, which can be used in rocky mountainous watersheds for erosion control and watershed management. [ABSTRACT FROM AUTHOR]

This study used a two-dimensional diffusion hydrodynamic model for the simulation of extreme flood events that occurred in the Shihmen reservoir watershed from 1989 to 2001. The alternating direction explicit scheme (ADE) with various horizontal spacings from 120 to 240 m at an incremental interval of 40 m was conducted for flood simulations. Spatial precipitation was provided by the gauge interpolations and weather radar rainfall estimate schemes. A case study, Typhoon Nari on September 16-18, 2001, was then performed using radar-derived rainfall with coupling two-dimensional diffusion hydrodynamic model in flood routing investigations. Numerical results revealed acceptable agreement between the observed and simulated reservoir stage hydrographs. Results of 120 and 160 m spacing had similar values on error indicators while the 160 m grids can greatly reduce computational time by 40% than that of 120 m grids. Precipitation is identified to be the main factor forcing model result. [ABSTRACT FROM AUTHOR]

The article discusses a study which examines a physical based distributed-parameter model combining surface runoff and ground water flow. A proposed watershed model for the simulations of extreme flood events and long-term periods that occurred in the Shihmen reservoir watershed in China from 1986 to 2001 was used to identify physics-based multiple processes occurring in watershed, including evaporation, infiltration, percolation, surface runoff and ground water flow. The study revealed that the proposed model exhibited a favorable potential for storm based simulations, recession period modelling, and long-term simulations.

In this study, we examine a physical raster-based distributed-parameter flood simulation model combining one-dimensional (1D) channel flow and two-dimensional (2D) overland flow. Continuity equations based on the water budget concept and momentum equations based on Manning's formula are included. The Shihmen reservoir watershed, situated in northern Taiwan, is selected as the study site, and data regarding the passage of Typhoons Xangsane [2000] and Nan [2001] are utilized to test the model. Comparative analysis shows that the description of the riverbed obtained by the gradient method is superior to that obtained by the GIS approach. This study suggests the Thiessen polygon method to interpolate spatial precipitation. The best calibrations are obtained at a spatial resolution of 160 m x 160 m, with a simulated time step of less than 5 s. The case study indicates that the proposed model has a good potential for inflow description, but does not accurately simulate the inflow amount. The proposed model is a flash reacting event-based watershed model. To improve simulation accuracy, a good infiltration model and soil moisture model are needed, and their formulation could be our next task. [ABSTRACT FROM AUTHOR]

Precipitation data is generally collected from rain gauge stations. However, each measurement represents only the amount of rainfall at that particular spot, not precipitation in the surrounding area. Radar approaches are considered to offer a good spatial description of precipitation, but hardly predict precipitation quantities with acceptable accuracy. To enhance our understanding of the properties of precipitation in this study, high resolution radar-rainfall estimates are compared with ground observations for an extreme precipitation event. The Taipei City area and the Shihmen reservoir watershed, situated in northern Taiwan, were chosen as the study sites, and the passage of Typhoon Nan through these areas on September 16-18, 2001, was taken as the case study event. It was concluded that radar reflectivity from the Wufenshan radar station can be helpful for identifying precipitation variations during the passage of a land falling tropical cyclone. Spots with extreme rainfall can be identified when radar approaches are performed, but not based on gauge approaches, although compared to the gauge approaches to the radar-estimated rainfall over the investigated domain tended to be overestimated. The divergence between radar-rainfall and gauge-rainfall can be identified via subwatershed investigations. [ABSTRACT FROM AUTHOR]