Long Term Hydrological Monitoring Program at the Fushan Experimental Forest in Taiwan

Yue-Joe Hsia

Institute of Nature Resources

National Donghwa University

Shoufang, Hualien

Tel: 886-3-8662500 ext. 1738

Fax: 886-3-8662533

E-mail: yjhsia@cc.ndhu.edu.tw

Abstract

Rapid industrialization and urbanization in Taiwan since 1960s have caused the natural resources and environment seriously degraded. Specifically, a large area of the low to mid altitude subtropical, mixed evergreen forest classified as mixed forest stand and accessed litter commercial timber value by the Taiwan Forest Bureau has been converted into tea, fruits, biter nut, and pure stand of conifer plantation. However, the loss of this natural forest covers represents a serious threat to the functions of the ecosystem and the quality of water resources. The problem has aroused scientists, governmental agencies, and general publics concern. Yet, no effective management practices have been proposed because the lacking of enough knowledge on the structure and function of this type forest ecosystem.

The Fushan Experimental Forest is located 40 km southeast of Taipei in northern Taiwan and is managed by the Taiwan Forestry Research Institute (TFRI) since 1990. Elevation of the forest ranges from 400 m to 1400 m and represents the typical moist, subtropical, mixed evergreen forest of the NE Taiwan. The area is relatively undisturbed and has been selected as one of the Long-term Ecosystem Research (LTER) site by the National Science Council, Taiwan since 1993. One of the core research goals at the Fushan site is to understand the hydrological and biogeochemical processes of this moist, subtropical, mixed evergreen forest. Two small gauged watersheds (Fushan watershed No. 1, FS1 and No. 2, FS2) were established as primary research sites. In order to maintain a reliable long-term project, the monitoring program has been kept simple and with an eye on manpower savings. Automation is also considered as a critical prerequisite for all data collection programs. Hydrological and meteorological data are all recorded by data logger and processed by a computer package. The computer packaged is tested before the field program started. Currently, the database is readily available to all researchers working at Fushan.

Average annual rainfall at the Fushan forest was 3700 mm. Rainy day was 221 days annually. Monthly rainfall averaged more than 200 mm from April to November. Heavy rainfall accompany with typhoons dominated the rainfall distribution from June to early October. Frontal systems in the winter sometimes can also bring heavy rainfall events too. Four climatological stations are maintained in the Fushan Experimental Forest. Two of them are ground stations, which provide long term climate records for the whole area. The other two climatological stations are mounted on instrument masts at 20 and 30 meters heights respectively to monitor the above forest canopy climate. In addition to the wind, temperature and humidity, global solar, reflected solar, net and longwave radiations are also monitored. All parameters are recorded hourly with solar powered data logger. As an example for the simplicity consideration, the pan evaporation measurement is not made at Fushan because the difficulties of maintenance of a clean evaporation pan as well as the need of field attendance. . Instead, potential evapotranspiration is calculated as a reference instead. The two watersheds (FS1 and FS2) are gauged with 90o V-notch and rectangular weirs to accommodate both low and high discharge events. Gauging weirs were constructed using watertight type RC boxes. After the FS1 weir was completed in 1992, seepage below the weir was observed. To help to remedy the problem, a 30 X 60 m heavy plastic sheet was placed upstream of the weir. Due to deep bedrock fracturing at the gauging site, deep seepage below the weir still exists. To prevent deep seepage, deep boreholes were drilled and pressurized cement inserted prior to the FS2 weir construction. Stage heights at the gauging stations are recorded in 5-minute intervals. Flow proportional composite water samples are taken weekly and bulk precipitation is collected on a storm base schedule.

The preliminary analysis of the data collected over the past five years indicated that the hydrological characteristics at the Fushan Forest is indeed marked by the torrential rainfall brought by typhoons. Potential evapotranspiration calculated with the Penman, Harmon, and Thornthwaite methods are all lower than 900 mm annually. The actual evapotranspiration is close to the potential evapotranspiration. Analysis of stormflow indicates that stream flow responses quickly to the storm rainfall. For the more frequent small storm, the ratio of stormflow to net rainfall is small but for storm events with rainfall greater than 100 mm, the ratio reaches more than 50%. The contribution area of stormflow for small storm, therefore, generally is confined to the narrow channel valley and the area extends up to hill slope for those large storms. The deep incised channel steep slope, short hill slope length, and high infiltration rate contribute to that subsurface runoff is the main mechanism of stream flow generation during storm periods. Annual runoff ratio at Fushan Watershed No. 1 (FS1) averaged around 50% whereas the ratio reached 80% at Fushan Watershed No. 2 (FS2). The lower runoff ratio at FS1 is mainly caused by the deep seepage in the fractured bedrock at the FS1 gauging site. However, ratios of stormflow to the precipitation at the two watersheds are similar (in 1997, 39% and 36% for FS1 and FS2, respectively). Since the role of typhoons in the nutrient budget and other ecosystem structure and functions is significant and more process studies are needed.

Key Words: typhoon, forested watershed, LTER

Table 1. Storm characteristics of typhoons at Fushan watershed No.1 (FS1) and No.2 (FS2).

        Year Name of typhoonRainfall, P, mmStormflow, Q, mmQ/P, %Peak flow, liters/s/ha

FS1  1994 Tim  425  140  35  20.3

            Caitlin  160    20  13  5.8

             Doug  375  190  50  16.0

              Fred  340  150  45  17.9

          Claidys  250    90  35  37.0

              Seth  720  385  55  37.2

       1996 Gloria      105  10  10     2.0

                  Herb     775  480  30  57.7

       1997 Winnie     480  245  50  22.5

               Amber     660  340  50  53.2

FS2  1996 Gloria  120    30  25  3.9

                  Herb  810  680  80  78.3

        1997 Winnie  525  345  65  44.4

                Amber  680  450  65  78.4

Table 2. Streamflow characteristics of Fushan Watershed No.1 (FS1) and No. 2 (FS2).

YearRainfall, mmStreamflow, mmTyphoons

TotalStormflowBaseflowStormflow of rainfall events > 100 mmRainfall, mmStormflow, mm

FS1  1993  3000  1510  240  1270  180  0  0

           (50%)1  (16%)2  (75%)3

        1994  4900  2700  1220  1480  1160  2270  970

           (55%)1  (45%)2  (95%)3  (46%)1  (80%)3

        1995  2850  1310  310  1000  270  0  0

           (46%)1  (24%)2  (87%)3

        1996  4570  2270  1120  1150  1030  890  490

           (50%)1  (49%)2  (92%)3  (19%)1  (44%)3

        1997  3530  2040  790  1250  740  1090  580

           (58%)1  (39%)2  (94%)3  (31%)1  (73%)3

FS2  1997  3700  3170  1140  2140  1030  1200  800

           (86%)1  (36%)2  (91%)3  (33%)1  (70%)3

1) percent of annual total rainfall

2) percent of annual total streamflow

3) percent of annual total stormflow