A new biosphere hydrological model, so-called Water and Energy Budget based Distributed Hydrological Model (WEB-DHM), has been developed by coupling a biosphere scheme (SiB2) with a Geomorphology Based Hydrological Model (GBHM).

In WEB-DHM, the improved SiB2 incorporating descriptions of sparse canopy processes, describes the transfer of the turbulent fluxes (energy, water, and carbon) between atmosphere and land surface for each WEB-DHM grid; while the GBHM redistributes water moisture laterally through simulating both surface and subsurface runoff using grid-hillslope discretization and then flow routing in the river network using the kinematic wave approach. The subgrid parameterization of WEB-DHM represents topographical characteristics using fine resolution DEMs, while the model keeps high efficiency for simulations in large-scale river basins by simplification of the streams in the large model grids. After model development, model validations and further studies were carried out step by step.

Firstly, the WEB-DHM was validated in Upper Tone River Basin of Japan (a humid region with long-term mean annual precipitation around 1500 mm) and Yongding River Basin of China (a semi-arid region with long-term mean annual precipitation about 400 mm), which supply water for Tokyo and Beijing, respectively. The applications to the two basins with different climate are detailed as follows.

(1) In Upper Tone River Basin, high flood risks come from heavy rainfall events occurring from June to October, which are commonly associated with typhoons and Mei-yu front activities. Several reservoirs were constructed for protecting the Lower Kanto plain from floods. Based on 3-year (2000-2002) meteorological data, the model was calibrated at 2000, and validated from 2001 to 2002. Comparing to GBHM, the new model is more physically-based in describing land-atmosphere interactions and estimation of evapotranspiration (ET), and thus has less parameters to be calibrated. Good results have been achieved in simulating hydrological processes and water budget. Hourly hydrographs showed the model's prediction capability of floods, including the ones after periods of low water flows. Water budgets analysis has demonstrated the model's accuracy in estimating long-term ET. Simulated annual largest flood peaks matched well with observed ones in both flood peak and flood time.

(2) The semi-arid river basin, Yongding often suffered from water shortage. For improved water resources management, hydrological recovery is necessary for estimating the total natural surface water resources. In this study, a new approach for hydrological recovery is presented. Observed precipitation and other meteorological data sets from JRA-25 reanalysis project were used to drive the biosphere hydrological model (WEB-DHM) for hydrological recovery, through a complete simulation of natural hydrological processes using physically-based governing equations. Good performance in simulating annual natural surface water resources has been shown from the two-year (1990-1991) applications.

Secondly, the sensitivity of river runoff to input radiations was investigated in Agatsuma River Basin. The WEB-DHM was driven by different radiation inputs from empirical model estimates and/or reanalysis data for this sensitivity study. In traditional hydrological studies, the input radiations were used to estimate potential ET empirically, but the accuracy of input radiations was not highlighted as precipitation. However, the solar and longwave radiation incident at land surface are crucial for hydrological cycle, since they determine the radiation budget, which, in turn, modulates the magnitude of the terms in the surface energy budget (e.g., ET). The results showed that river runoff was sensitive to magnitudes of input radiations in the humid region, indicating the importance of the accurately estimating input radiations.

Thirdly, the WEB-DHM has the potential to give projections of future water resources considering the dynamical response of river runoff to rapidly rising CO2. Much attention is paid to climate change-induced precipitation patterns and land evaporation alteration, while the CO2-induced transpiration adjustment received less attention. As a result, most projections of future water availability have tended to neglect stomatal-closure effects since most of current DHMs did not consider the direct CO2 effect on river runoff due to lack of descriptions of biospheric processes. In the Agatsuma River Basin, the biosphere hydrological model (WEB-DHM), incorporating a canopy photosynthesis-conductance model to describe the simultaneous transfer of CO2 and water vapor into and out of the vegetation respectively, was used to investigate the direct CO2 effect on vegetation transpiration and thus runoff quantitatively, while neglecting vegetation structural feedbacks to CO2 increases. Although the countering direct effect of increased photosynthesis and resulting increases in biomass (and LAI) from CO2 enhancement has not been yet incorporated into WEB-DHM, however, the work was one of the earliest explorations in CO2-induced variability of streamflow using a biosphere hydrological model.

Through validations in both humid and semi-arid river basins, WEB-DHM has shown good performance in simulating both single flood event and long-term continuous hydrological processes. Therefore, the model can be used for flood prediction as well as long-term water resources estimation. Sensitivity study on input radiations showed the significant differences between simulated river runoff from various radiation products. It indicated the urgency to improve radiation products in Predictions in Ungauged Basin (PUB), where the calibration could not be finished. The evaluations of direct CO2 effect on river runoff due to vegetation physiological feedback to CO2 increase, has shown the potential of WEB-DHM for future water resources projections under a CO2 changing climate.

豪雨災害,水不足,水質汚染,生態系の破壊など水に関わる深刻な問題が世界各地で近年広がってきており21世紀は水危機の時代といわれている.これらの問題は,人口増や都市化などの社会的諸要因を有する地域で水循環の大きな変動が生じた場合に一層深刻となる.水循環変動のメカニズムを理解し,その予測精度を向上させる科学的基盤を形成することは,水危機回避の有力な解決策の一つと言える.

水循環はエネルギーフローとあわせて地球気候システムを形成する重要なサブシステムであり,全球規模,地域規模の水循環が流域規模の水文現象と密接に関係している.つまり,流域規模の降雨や河川流出は全球規模,地域規模の水循環変動の影響を色濃く受けている.したがって,たとえ河川流域規模の水問題に対応する場合にも,全球規模,地域規模の大気の水循環変動予測情報を効果的に河川流出予測情報に結合させる手法の開発が急務となっている.そこで、本研究は,数値気象予測モデルと分布型流出モデルとを物理的に結合する手法に開発に取り組んでいる。

本研究では、河川流出モデルに組み込まれている各グリッドでの水の鉛直、水平フローを表す機能と、大気-陸面結合モデルに組み込まれている陸面スキームが有する機能とを兼ね持つ新たなスキームを開発した。陸面スキームとしては多くの数値気象予測モデルで利用されており、全地球規模の展開可能なスキー(SiB2)を、河川流出モデルとしては流域斜面での地表水、土壌水、地下水の挙動を物理的に表現できる分布型流モデル(GBHM)を採用しており、汎用性を有している。本研究で開発されたモデルの特徴は、地表面での水収支のみならずエネルギー収支を分布型流モデルのみの場合に比べてより正確に算定するため、流域からの蒸発散や土壌水分分布を精度よく表現していることが、わが国の利根川や中国の永定河へのモデルの適用と水収支解析によって示された。さらに、長期の無効期間後の洪水の再現や、低水流出の再現性において、再現性に優れており、年間を通して,低水から洪水まで河川流量を一貫して表現できることが示された。

本結合モデルの利点の一つは、陸面スキームのパラメータ特性を、河川流出量という積分値(観測可能な値)で明らかにすることができる点である。これはポイントでは直接観測から可能であるが、流域内の分布情報を得ることは不可能に近い。そこで本研究では、日射量の推定誤差の感度や、温室効果ガスの増加に伴う水循環変動特性(二酸化炭素濃度の増加とともに、気孔調節によって蒸散量が減少し、河川流出流が増加する)を数値シミュレーションによって明らかにしている。

以上本研究は,地表面での水・エネルギーフローと洪水や渇水など水循環変動を定量的に算定するという科学的側面だけでなく,気候,気象,水資源,農業,生態系などの社会的利益分野にも貢献するところが大きく,科学的,社会的有用性に富む独創的な研究成果と評価できる.よって本論文は博士(工学)の学位請求論文として合格と認められる.