Linking runoff process and spatial and temporal variation of water chemistry in a forested headwater catchment 森林源流域における流出過程と水質の時空間変動との結合に関する研究
Linking runoff process and spatial and temporal variation of water chemistry in a forested headwater catchment
Quantifying the change of water chemistry as a function of time-dependent contribution of pathways during runoff process is critical to understand the chemical varaiability and to conceptualize properly solute transport through the hydrologic pathways in a catchment. The present study deals with the linking runoff process and hydrochemical changes in space and time across the hillslope segment in a forested headwater catchment.A first order drainage basin of 5.2 ha from the total area of 14 ha of the Kawakami Experimental Basin (KEB) situated in Nagano prefecture, central Japan was used for the experiment. This forested headwater catchment located at an altitude of 1500 m to 1680 masl with slopes vary from 20 to 60%. This area underlied by Late Neogene of the Meshimoriyama volcanic rocks with primarily consists of Inceptisols/ Mean annual precipitation ranges from 1500 to 1600 mm, producing 800 to 900 mm of runoff. A natural deciduous forest of oak (Quercus mongolica Fisch), larch plantation (larix leptolepis Gordon) and the bamboo grass (Sasa nipponica) cover the entire catchment.Combined hydrometric and geochemical approaches were applied to define the linkagemechanism of flowpaths dynamic during runoff process with hydrochemical changes in a transect across the northern valley, where the riparian zone was prominently developed. The cross section consists of 12 observation nests with 57 posits of geochemical measurement and 4 points of stream chemistry. End-member mixing analysis (EMMA) was performed to predict the proportion of stream water from three principal source components of storm runoff on August 21-22, 2001, the snowmelt runoff and the rain-snow runoff on March 28-29, 2001.Subsurface flow was identified to be the major hydrologic factor generating runoff in Kawakami headwater catchment. Three components hydrograph separation of the storm runoff predicted by EMMA using Ca^〈2+〉 and SiO_2 showed that the near surface riparian (NSR), the hillslope soil water (HSW) and the deep riparian groundwater (DRG) are the major sources of runoff, which contributed as much as 45%, 35% and 20% of the total storm runoff respectively. The analysis using Ca^〈2+〉 and SO_4^〈2-〉 resulted as much as 55%, 23%, and 22% respectively during the snowmelt event and 73%, 12%, and 15% respectively during the rain-on-snow event. The NSR water dominated the storm runoff during baseflow (87%), early on-set rain (58%), storm end (66%) and post storm (76%), whereas the HSW was dominance during the period of around the peak (41-52%) and the late falling limb (59%). The DRG had never dominated the storm runoff even at the peak flow. As much as 143.5mm of total rainfall, 42% of it quickly became derect runoff from the catchment. The development of antecedent wetness in the hillslope soil mantle creates a threshold response of rapid delivery mechanism of hillslope soil water to reach the stream channel. There was no significant overlend flow occurred in this catchment event during the large storm.The water chemistry varied in space and time with response to the runoff process. The defference in frow magnitude and direction between the source areas of runoff was consistence with the spatial variation of the solute concentration. The solutes concentration of the riparian zone was higher than that of the hillslope segment. The spatial variation of the water chemistry was also controlled by the solutes residence time (Tr). Expect for Na^+ and Cl^-, the solutes residence time obey the - following sequence: Tr_〈DRG〉 > Tr_<NSR> > Tr<HSW>. Time-dependent changes of flowpaths under varies rainfall amount creates a temporalvariability of water chemistry.The change of flowpaths during the storm runoff coincided with the change in the streamwater chemistry. The hillslope floepaths more pbviously changed than the riparianflowpaths. Large increases in discharge in response to storm as the result to the development of the antecedent wetness, which changes the floepaths from the relatively vertical at the basefloe period into the shallow lateral down slope at the peak storm is responsible to the change in Ca^<2+>, SiO_2, SO_4^<2-> and other several solutes concentration in the stream water. The stream water chemistry did not change considerably during the snowmeltevent.The dinamic of solute transport was clearly inserred from the magnitude and the vzriation of solute fluxes in space and time. Solutes flux decrease during low flow and increase when the flow was high. Except for NO_3^-, the solutes flux increased during the rising limb and decreased during the fallinglimb. Ca^<2+> has higher flux compared with other solutes.The importance of the NSR zone on runoff generation and stream chemistry is defined by (a) the largest contribution to total runoff, and (b) the reatively steady lateralflows that could facilitate flushing of high ssolutes concentration. In contrast, the contribution of the DRG was small with the relatively steady downward flow. This phenomenon is in conflict with the typical assumption that the whole riparian zone may reset flowpaths and chemicalsignature of hillslope water. The rapid response of the HSW to the stream chemistry during the peak storm suggests that the hillslope flowpaths be hardly restricted by the NSR flowpaths.The conceptual model of the linking runoff process and hydrochemical changes for Kawakami headwater catchment is proposed. Two distinct units governing the runoff and solutes processes are included: (a) the link near riparian reservoir and hillslope aquifer, which coontrools rapid flow and solutes flushing during the hydrologic event and (b) the deep riperian groundwater reservoir, controlling predominantly slow flow and serving a rechsrge system and solutes and accumulation. In addition, 4 destinct zones are involved in producing the hydrologic and solutes variability in space included (1) a zone with rapid flow, fast depletion of soil water and groundwater, and flushing of high concentration of solutes (zones 1 ), (2) zone with variable flowpaths, shallow groundwater level and intense solute mixing (zone 2), (3) zone with delayed response to stream, slow change of flowpaths and high accumulated solutes (zone 3), and (4) zonewith fast infiltration, repid depletion of soil water and repid changes of flowpaths and water chemistry (zone 4). A distinct pattern of hillslope flowpaths occurred during snowmelt and rain-on-snow event allowing to differentiate the model.
Thesis (Ph. D. in Science)--University of Tsukuba, (A), no. 3124, 2003.3.25
Includes bibliographical references
森林源流域における流出過程と水質の時空間変動との結合に関する研究 ~ カズディ、スバギョノ