A basic analysis of the characteristics of the time series fluctuation and the spatial distribution of rainfall on Mt. Merapi in Indonesia and Mt. Fuji in Japan is presented in this report. In the for rid zone, precipitation is much more variable over time and space than in the temperate zone. Therefore, it is important to know the relation between deviation power and spatial scale when es timating hydrologic quantity in the torrid zone.
The rainfall characteristics of Mt. Merapi were compared with those of Mt. Fuji to clarify the difference in precipitation between the two climatic zones. Next, spatial rainfall data for Mt. Merapi was interpolated among five ground rainfall stations and linked with the spatial data obtained by the radar rain gauge system to estimate the interaction between the time scale and spatial scale of precipitation. As a result, it became clear that varying the time scale from 1 hour to 1 month enhanced the spatial scale that could be represented by point rainfall data. In addition, the spatial fluctuation of rainfall for 1 month on Mt. Merapi was found to be equivalent to 1 hour on Mt. Fuji.

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The evaluation method for evapotranspiration and its application to the actual area of pyroclastic deposit on the south-western slope of Mt. Merapi volcano are presented in this paper. In the study sites (Gunung Maron site at an altitude of 960m and Pusung Malang that of 1200m), it can be assumed that the amount of evapotranspiration can be evaluated by two processes. One depends on the transpiration by plants which is related to the water absorbed from the soil through their roots, the other is due to direct evaporation from the ground surface. We can evaluate the water balance for a soil layer by adding; the amount of water which infiltrates from the ground surface during rainfall, gravitational water moving down and passing through the layer, evaporation water through the ground surface and transpiration water by plant leaves. On the basis of this idea, an unsaturated water flow simulation is carried out to evaluate the amount of evapotranspiration by using soil suctions observed at two sites. The results show that the annual amount of evapotranspiration from August 1992 to July 1993 is about 1600mm at Gunung Maron, and the seasonal amount from August to November 1992 is about 500mm at Pusung Malang.

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Mt. Merapi is an active volcano in Indonesia and has several watersheds in its southwestern slope, e. g., the Blongkeng, the Putih, the Batang and the Bebeng watershed. The evapotranspiration in this region was estimated whose spatial and temporal resolution was 50× 50m and one month, respectively. The method was based on the Penman Monteith Equation, which requires the information on five elements, i. e., effective radiation, air temperature, vapor pressure, aerodynamic resistance and canopy resistance. Using air temperature data observed by the Indonesian Volcano Observatory, the relationships between altitude and air temperature were approximated by linear lines. Effective radiation was calculated with the result of numerical analysis about the shadow of the mountain and remote sensing analysis using GMS ISCCP-B2 and SPOT. Aerodynamic resistance was calculated from the representative vegetation heights based on the ground truth. Canopy resistance was calculated with NDVI from the data of satellite SPOT. The area at low altitude had a higher value of evapotranspiration than that of mountainous areas. The annual amount of evapotranspiration was 1, 210mm in the Putih watershed, and was approximately 1, 400mm in the other three watersheds. Every watershed had a high peak on September, a small peak in April and the lowest value in January. The lowest evapotranspiration rate each month was recorded at the Putih watershed. The Bebeng watershed had a high evapotranspiration at the altitude of 800 through 1, 000m, while the Putih watershed had no such high peak. The Bebeng watershed had a high peak of NDVI at the altitude of 800 through 1, 000m, which correspond to high evapotranspiration. The difference of vegetation between these watersheds was concluded to have caused the difference of evapotranspiration between these watersheds.

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Direct runoff characteristics in several streams on the southwest slope of Mt. Merapi were identi fied, and the effects of catchment factors on the direct runoff characteristics were discussed distin guishing surface runoff and subsurface runoff. Stream flow data was taken from 6 observation points on 4 rivers. Direct runoff was separated from total runoff in each storm hydrograph. An exponential relationship was shown between rainfall and direct runoff for each storm event. There was a great amount of surface runoff just after the 1984 pyroclastic flows. Subsequently it de creased gradually because of restoration of infiltration capacity on the mountain slope. Currently direct runoff is thought to consist of surface runoff which rises in rice fields and channels, and sub surface runoff which rises in shallow groundwater. The amount of surface runoff in relation to rainfall could be estimated by the percentage area of total rice fields and channels based on certain assumptions. Subsurface runoff estimated from surface runoff could be accounted for by estimated shallow groundwater movement from 400m to 600m above sea level.

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The Water balance on the south-west slope of Mt. Merapi in 1993 is made clear in this paper. The catchment in question is composed of four main rivers and several other small streams. The reason why so many rivers and streams were bundled together for analysis of the water balance can be found in the fact that all the rivers or streams have quite distinct runoff and it is difficult to ascertain the domain of the phreatic basin. Precipitation, evapotranspiration and runoff into and from the total subject catchment and each sub-catchment were estimated independently with data by direct measurement or by appropriate calculation. According to the calculation on the water balance, deep seepage disappearing out of the total subject catchment was as much as 1, 094mm. This value implies the existence of a huge amount of groundwater. The delivery of groundwater among sub-catchments is discussed for each of water balance, spatial distribution and characteristics of fluctuation of groundwater tables and the topography of sub-catchments. Consequently, the hydrological uniqueness of each sub-catchment and the complicated feature of groundwater in the volcano become clear.

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Hydrological variables were measured for three water years at the southern slope of the Bandai Volcano. Annual percolation rates into deeper layers were from about 700 to 1000mm year^-1, and most of the annual precipitation was consumed by the percolation. In comparison with the water balance analysis at the other volcanos, it was pointed out that the percentages of the annual percolation rates to the annual precipitation decreased with the increase of the dissection of valleys. A time series of the percolation into deeper layers estimated by the short-term water balance method showed the monotonous variation. The peak rate occurred in June just after the sonowmelting seasons and the minimum values during a winter. The tendency of the variation on the percolation agreed with the fluctuation of groundwater levels in the study area.

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Runoff characteristics on volcanic areas were investigated by using a conceptual model (HYCYMODEL) . The HYCYMODEL was applied to four watersheds in Mt. Merapi, Mt. Bandai, and Mt. Iwate, and the runoff characteristics were evaluated as parameters of the model. Percolation into deeper layers, which is typical in volcanic areas, was expressed as a function of base flow discharge. A parameter in the storage-runoff function of base flow in volcanic areas was different from that of non-volcanic areas, indicating that the base flow discharge in volcanic areas decreases very slowly. Also, a parameter expressing the storage of subsurface layer was large. From these results it is suggested that volcanic areas have a large water storage capacity. But the degree of percolation into deeper layers differs greatly among volcanic watersheds, being affected by the topographic degree of dissection on each volcano.

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The erosion rate of the accumulated sediment in gully channels is described based on the morphological analyses of the cross sections of the gully channels. Three measuring values of the gully channels were used for the analyses, these were the sum of the cross sectional area, the depthwidth ratio of the cross section and the cross sectioned profiles of the terrace deposits.
The long term erosion rate of Mt. Merapi in the past one thousand years is shown in the relationship between the duration without pyroclastic flows (T) and the sum of the cross sectional area of the remnants from the pyroclastic accumulation on the channel bed (Vr), and is as follows, V_r=3.4·10⁵exp(-4.9·10^-3·T)
In the most recent erosion rate during the last 60 years, the depth-width ratio of the cross section has decreased according to the increment of the frequency of the pyroclastic flows. The gully channels with pyroclastic accumulation of more than once in 15 years were thought to be filled by enormous sediment. The erosion rate after the pyroclastic flow in 1984 is shown in the channel bed changes, that is the boundary point between the aggrading reach and the degrading reach has moved back about 2km to the upper reach during the last 8 years.

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Merapi volcano located in the central part of Java, is one of the most active volcanoes in Indonesia and has had repeated volcanic eruptions with avalanche type pyroclastic flows caused by collapsing lava mass. Recently, larger scale pyroclastic flows occurred in June 1984. The pyroclastic flow deposits widely and deeply covered the upper reaches of the Putih and Bebeng rivers and created a radical alteration of the hydrological and erosion regime of the basins. As a result of that, much sediment was produced by sheet-rill and gully erosion from the hillslopes and was transported by volcanic debris/mud flows and floods to the lower reaches of the Putih and Bebeng rivers.
In this paper, the total amount of sediment yield by sheet-rill and gully erosion on the hillslopes covered with pyroclastic flow deposits in 1984 was evaluated and the rates of the sediment yield were estimated, based on a field survey and interpretation of the two sets of aerial photographs taken in 1984 and 1991. The total amount of sediment yield was estimated to be approximately 2. 2 million m³ for the Putih river and 4. 0 million m³ for the Bebeng river between the eruption in 1984 to 1991. It seems that most of the sediment was produced in a 4-year period from November 1984 to October 1988. The rate of sediment yield rapidly increased to a peak between November 1984 (5 months after the 1984 eruptions) and October 1985, and then slowly declined with time keeping a high level until October 1988.

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Merapi volcano which has often erupted with pyroclastic flows is one of the most active volcanoes in Indonesia. Recently, large-scale pyroclastic flows occurred in the southwestern flank of the volcano in June 1984. As a result, the hydrological and erosional regime of the hillslopes was radically altered and more than 203 debris flows and floods have occurred in the Putih river catchment.
In this paper, using the records of debris flow in the Putih river catchment, the characteristics of debris flow following the 1984 pyroclastic flows were analyzed. The rainfall conditions not causing debris flow are definitely different between the 4-year period after the 1984 pyroclastic flows and since then. The rainfall intensity not causing debris flow is small just after the pyroclastic flows and then has increased with time. Also, the large-scale debris flows occurred within a 4-year period after the 1984 pyroclastic flows. The magnitudes of debris flows have decreased with time. The sediment outflow by debris flows had almost finished within a 4-year period after the pyroclastic flows.

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Observation of storm runoff, suspended sediment and debris flow, and topographical surveys of the channel at the Bebeng River on the southwestern slope of Merapi volcano for 3 years since 1991, have clarified the characteristics of the low runoff coefficient for storm runoff, the large flux of suspended load, and the conditions of debris-flow occurrence, motion and deposition. Estimated annual sediment discharge through the observation site consists of 167, 000m³ of debris flows and 47, 000m³ of total load. Comparison of sediment transportation at three volcanic torrents on Mount Yakedake, Mount Unzen in Japan and Mount Merapi indicates a trend where the frequency and scale of debris flows, and the amount of sediment yield at a specific volcanic torrent generally decreases exponentially with the time after the last effective eruption. For example, annual depth of sediment yield or sediment discharge might be expected to decrease from the initial rate of 10¹-10²mm/year following the eruption to 10¹mm/year in less than several years, and finally approach 10⁰mm/year in less than a few decades.

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A pyroclastic flow occurred due to the eruption of Mt. Merapi in 1984, the debris was deposited widely on the south western slope from the top down to about 900m in altitude. There were several disastrous debris flows which occurred over the next few years, and the upper reach of the Bebeng River was deeply eroded. In this paper, the erosion process over a period 6 years, after the 1984 pyroclastic flow on this river, is simulated by using a one-dimensional mathematical model. From this analysis it was estimated that the lowering rate of the river bed per year in the upper stream of the Bebeng River due to erosion after pyroclastic flow deposit was about 2m/year for 2 years just after the event, and about 1m/year after for 4 years.

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