## 5 Results
In the surrounding area of TAIS, some gullies were observed, but there was no obvious water system developed, which was not conductive to concentrate runoff. A similar topographic condition was observed in the vicinity of the TSUN station. This might be the reason why the rainfall decay time of these two stations persisted longer than the others because the rainwater was slowed down by the rugged terrain. Nevertheless, not all BSMs installed in the hill or mountain area show a long persistence in response, e.g., BMMT. BMMT is located on a terrace top in the hilly region with a river 250 m west of the station; the altitude is approximately 195 m. The responding areal strain of BMMT reached a peak of 5.23 nanostrain/hPa, and gradually declined over time in half hour after the rain started. It is notable that the rainfall decay time of BMMT did not last as long as the BSMs in hilly area such as TAIS and TSUN. Since the network of streams will affect the disputation of surface runoff [48; 49], the river meandering around the terrace where BMMT sit might be one of the important factors for dispersing over-land water. In addition, we found some significant features in DARB. Taiwan is located on the plate boundary and experiences a dramatic rise of strata under the mutual compression of the plates. Within the distance of about 100 km, the terrain has risen to thousands of meters high from the west coastal plain to the central ridge. It is therefore difficult to avoid the installation of borehole strainmeters on high mountains. DARB is located on a terrace of the upstream of the Zengwun River in Southern Taiwan, with an altitude of 953 m and surrounded by a large number of mountains >1000 m. There are some rivers and creeks around the station. The observed strain data of DARB were highly sensitive to rainfall, and the strain values and variations differed from those of other stations. Thirty rainfall events from 2007 to 2012 were selected for rainfall impact function calculations (Figure 5). The results were concentrated in two ranges, relatively small responding strains below 2.0 nanostrain/hPa and higher strains above 2.0 nanostrain/hPa with a notable delay. Based on the strain distributions, we divided the data into two groups for the discussion: group I, smaller responses (twenty rainfall events); and group II, larger responses (ten rainfall events). The computed average rainfall impact functions for the two groups were plotted in the same coordinate system to distinguish the potential differences of both. Apart from the maximum responses, the duration values were also different apparently. The areal strain of group I rapidly reached the maximum responding strain of 1.53 nanostrain/hPa half an hour after the rainfall started and then declined over time. In contrast, the areal strain of group II reached a peak of 5.33 nanostrain/hPa 25 h after the rainfall starts. Both delay time and strain values of group II were higher than those of group I. They show a similar strain variation in the first five hours and rainfall decay time of the responding strain, persisting for 4.5 days, which may be associated with the area of catchment. In the previous session, we used a technique of deconvolution to estimate the individual rainfall impact function of BSMs. With stacking up the results of rain impact function obtained in a different time period, we could extract the average function for all events. In the case, it will strengthen the common and weaken the uncommon parts. However, the average functions often show discretely and noisily. Thus, we tried to find a function that could describe the behaviors of the most of average function in the continuous curve. In some studies, the runoff effect induced by rainfall could be fitted by an exponential curve under the limit of the power law [50; 51; 52; 53]. According to the results, the response will reach the maximum in 1-2 h, then show an exponential decay with time. It can be described as the following equation:  
\[h(k)=\left\{