休耕決策對灌溉用水影響之空間分析

Abstract

台灣雖有豐沛之降雨量，但因時間空間分配不均，缺水事件時常發生，且為了因應2002年加入世界貿易組織，將以輪作、休耕等措施來達到減產稻米的目標，故不管是在乾旱應變亦或是政府的農糧政策上，都可以發現以「休耕」來達到水資源調配與水稻減產的手段將日益頻繁。然而根據以往之研究，在休耕區域選定時雖已有初步之準則，但卻無法得知確實執行後水量變化的情況，故本研究針對不同休耕區域之空間分佈型態，探討休耕位置對於可節餘水率的影響。 研究中以曾文烏山頭灌區為研究區域，以各小組取水渠道層級與取水距離設定不同之休耕情境並進行隨機休耕模擬，以其模擬結果探討休耕面積與可節餘水率的關係，評估不同休耕區域之空間分佈對於灌溉水量的影響。最後利用空間自相關之概念，以空間分佈型態為基礎建立休耕節水效率評估指標，以該指標評估不同休耕之空間分佈對於可節餘水率的影響，免除在進行休耕規劃時繁複的水量計算，並提供相關單位進行休耕決策時的參考。 透過研究之結果可以發現，在休耕面積50%時因休耕區域不同所造成之可節餘水率變化最大，變異為14.7%。當中，休耕位置距離越遠且各小組間越團塊者可節餘水率越大，而距離越近休耕位置分佈越零散者可節餘水率越小。而休耕節水效率評估指標整合了渠道關係，改善傳統空間指標未考慮田區與渠道相互關係之問題，並令其計算結果更趨合理。綜觀而言，若能審慎規劃休耕區域，將可在相同的休耕面積下節餘出更多的水量，使水資源運用更具效益。

Although with abundant annual precipitation, Taiwan suffers from frequent drought due to the in-uniform rainfall distributions in temporal and spatial aspects. Due to the large share of water use for agriculture sector, fallow becomes an important measure for water shortage mitigation during the drought. It is also a mean to reduce the rice production in response to the produces imports since Taiwan’s participation to the WTO in 2002. It can be anticipated that fallow may become more frequent phenomena and important concerns in the future regional water resource management. Experiences for prioritizing the fallow sequences during the drought have been learnt from past researches, but the irrigation demand reductions from different fallow decisions are not readily available. This study therefore aims to discuss the impact of fallow decision on regional irrigation demand through spatial distribution consideration of the fallow farms. The command area of Chia-Nan Irrigation Association in southern Taiwan was chosen as the study area. A simulation model was built for irrigation demand estimation under scenarios of different cropping patterns including fallow. The irrigation group, with an area of about 150 hectares, is used as the basic decision unit. There are more than 650 irrigation groups in Chia Nan Irrigation Association. Each irrigation group is assigned with an attributes as the distance to the water supply source. Irrigation groups are also assigned an index showing its spatial grouping relation according to distribution canal systems. System simulation is done with the model to study the variations of irrigation demands with the spatial pattern of fallow paddy farms. The concept of spatial autocorrelation is used to establish an index for evaluating the effectiveness of irrigation demand reduction under different fallow strategies. The results show that the variations of irrigation are largest, as much as 14.7%, for different spatial fallow patterns when the fallow area is about 50%. When the fallow regions are farther away from the water supply source and are more aggregated together, the smaller the irrigation demands are. It is concluded that more water can be saved from the same fallow area if the fallow decisions are planned carefully. The proposed evaluation index for effectiveness of irrigation demand reduction can be used for evaluation of different fallow strategies at the planning stage and to improve the regional water management effectiveness.