Spatial variability of gross rainfall

I. INTRODUCTION

1.1. Background

Interception is a process where a part of rain water retained on a vegetated canopy surface and later evaporated back to the atmosphere. Interception plays an important role on rainfall redistribution both spatially and temporally. During rainfall event, the canopy acts like a sponge for the rain water, after rain water fills the storage capacity then additional rain water could drip to the ground surface. This storage capacity is the function of canopy and rainfall characteristics. For a similar depth of gross rainfall that hit a canopy, a low intensity of rainfall would be higher intercepted than a high intensity of rainfall. More rain water would also be intercepted in an intermittent rainfall than in a continuous rainfall. This is because the rain water reaches to the canopy in a longer period, providing more time for the evaporation process. Interception is calculated for a vegetated area and expressed as the proportion of gross rainfall. Other studies show a variable result of interception; Jackson 2000 found 10 of rainfall interception at Agroforestry system in Kenya while Schellekens et al. 1999 found 50 of rainfall interception in a lowland tropical rainforest at northeastern Puerto Rico. In a watershed, existence of vegetated area is of importance to extent water availability for the ecosystem and to prevent flood. Knowledge of interception would leads to a better understanding of hydrology cycle in this area where interception is one of the processes that reduce the water input. Neglecting interception loss would result in an overestimation of water input in a catchments area. By taking into account the interception loss would result in more precise information when assessing a catchment water balance and later could lead to a better watershed management. Previous study to assess the biophysical indicators of Cicatih watershed has been initiated by Pawitan et al. 2006, while this study focuses on assessing the hydrological function of the watershed which is about forest canopy ability to intercept rainfall. 1.2. Objectives The objective of this study is to identify the potency of natural montane forest in rainfall interception.

II. LITERATURE REVIEW

2.1. Spatial variability of gross rainfall

Gross rainfall distribution varies spatially, but it could be assumed similar for a relatively small area. To anticipate the variability, researchers installed several rain gauges in their study site such as Jackson 2000 and Chappell et al. 2001 while Schellekens 1999 used only one rain gauge. Many rain gauges allow researchers to get a representative data of gross rainfall but required extra effort for the maintenance. Some researchers put the rain gauge above the forest canopy such as Asdak et al. 1998a and Schellekens 1999 to avoid forest edge effect. Asdak et al. 1998a put rain gauge 15 m above ground surface when they investigated the intercepton on tropical rainforest in Central Kalimantan so that the angle between the rain gauges and the top of nearest trees was greater than 45 o to ensure no surrounding environment affect the measurement. Jackson 2000 used several rain gauges to measure gross rainfall during his interception study on agroforestry system in Kenya. The rain gauges were placed above the forest canopy and in an open space near the sampling area, and mean value from the rain gauges then used as the area’s gross rainfall. Schellekens 1999 used a single rain gauge placed above the canopy at 26 m on a scaffolding tower to obtain the gross rainfall, while Chappell et al. 2001 were using a network of 34 rain gauges located within 10 km 2 to investigate the spatial structure of above-canopy rainfall. Ford and Dean 1978 found that in a calm weather there was no difference between gross rainfall measured above tree within 20 m of the sampling area and those 300 m away at 3 m lower and 5 m higher than surface level of the sampling area. But during windy weather, rain gauge place in the sampling area above tree measured less rainfall than those at 300 m away. 2.2. The role of canopy characteristics in controlling the net rainfall Asdak et al. 1998a mentioned that in a relatively small area, throughfall was a function of the canopy covers and canopy structure. Leyton and Carlisle 1959 found larger throughfall as distance increase from the tree stem case study of Lawson cypress stand. It was also mentioned that because of 1 canopy drip, often throughfall was higher than rainfall in the open space. In contras, Ford and Deans 1978 found largest amount of throughfall in small area close to the tree stem case study of young Sitka Spruce Plantation. These show that throughfall variability was large caused by different canopy characteristics. This canopy variability should be taken into consideration when determining the troughs andor gauges placement in the sampling plot so that the value could represent as much variability as existed in the forest stand. From other interception studies Table 1, throughfall could vary from 50 – 90 of gross rainfall, which was obviously caused by the canopy variability. A conical canopy would have less throughfall than the broadleaved one because the thickness of a conical canopy would make rainfall that hit the top leaves to be dripped to the lower leaves. More water then required to saturate the whole canopy before the drops could reach ground surface. In contras, a broadleaved canopy was thin that would require less water to saturate its canopy and also makes water hit the top canopy surface could reach the ground surface in a shorter time as drip. The quantity of water required to saturate a dry canopy referred as canopy capacity. During storm event, some raindrops hit the canopy, retained temporarily on its surface, then evaporated back to the atmosphere or flow down as canopy drip. The canopy would retain variably quantity of water on its surface depends on its structure such as leaf size, surface character, and the nature of branching. Water would falls from the canopy as drip after the canopy get saturated, but some wind or turbulence might interference the retained water causing the canopy to retain less water and reduce the canopy capacity. Hall 2003 mentioned that for coniferous trees which have large leaf area index LAI but small canopy size, the wetting of the canopy determined by drip from upper leaves while for broadleaved trees with small LAI but large canopy size, raindrop size significantly influence the canopy capacity. Canopy capacity did not have a certain interval of values because it was depend on the canopy characteristics. As shown in Table 2, canopy capacity from forest stands could vary between 1 – 3.5 mm but there was a possibility that in other canopy this value could be less or more. Some forests which were dominated by broadleaved trees might have lower canopy capacity than the conical one and a single layer canopy would also have lower canopy capacity than the multi-storey one. It because in a broadleaved and single layer canopy, more canopy cover was exposed to the rainfall area so that less rainfall was required to saturate the whole canopy while in a conical and a multi-storey canopy, the same quantity of rainfall might only saturate the top leaves of the canopy but not the lower ones. Jarvis 2000 found that epiphytes existence did add the total interception on a forest stand, but he could not mention the proportion of interception contributed by those epiphytes. Despite raindrops that hit the canopy surface, other raindrops could reach the ground surface directly without ever touching the canopy, identified as free throughfall. The proportion of canopy gaps that allow rain water to reach ground surface without hitting the canopy first identified as canopy porosity. Its value varies between 0 - 1. High canopy porosity would indicate large gaps existence in the forest canopy that allows large quantity of rain water falls as free throughfall. Hall 2003 mentioned that free throughfall contributes to the net rainfall that occurred in a short time lag with the gross rainfall. This means that when a forest has larger canopy porosity then the time lag between gross and net rainfall at the beginning of a rainfall event would be shorter because of the free throughfall contribution. Another contribution to the net rainfall was from stemflow; part of raindrops that flow on the tree branches, accumulated on the trunk and finally reach the ground surface. Ford and Deans 1978 mentioned that stemflow was positively correlated with the crown projected area. Their study on young Sitka Spruce plantation found largest stemflow volume on tree with largest crown projected area. Both bark texture and height of the stem will also influence the quantity of stemflow reaching the ground surface Jackson 1975. The thicker and coarser the bark, more water was needed to saturate it before water can reach the ground surface. This will result in a longer time lag between rainfall and stemflow occurrence at the beginning of a rainfall event. In agreement with Ford and Dean 1978, Aboal et al. 1999 found that trees with the largest 2 crown area, not dominated by their neighbors, and with the smoothest bark, collect the largest volume of stemflow case study of laurel forest. However, stemflow contributes a relatively small portion to the net rainfall Table 2. Asdak et al 1998a found stemflow 0.3 of gross rainfall in the logged forest of Central Kalimantan. In this case, throughfall was controlling the net rainfall. The sum of throughfall and stemflow measured identified as the net rainfall, the actual rainfall that reach the ground surface, and mentioned as the proportion to the gross rainfall. Obviously, canopy characteristics leaf shape, size, surface character, and the nature of branching did have great influence on the net rainfall, besides the rainfall distribution itself. Table 1. Comparative result from other interception studies of gross rainfall Study Site Location T f S f I Source Unlogged forest Indonesia 87.2 1.4 11.4 Asdak et al. 1998a Logged forest Indonesia 93.5 0.3 6.2 Asdak et al. 1998a Agroforestry system with no intercropped Kenya 88.4 0.7 10.9 Jackson 2000 Hardwood stand Canada 76.4 4.3 19.3 Carlyle-Moses et al. 1999 Natural forest Indonesia 72.3 1.7 26.1 Anwar 2003 Lowland tropical rainforest Puerto Rico - - 50 Schellekens et al. 1999 Tropical upland mixed cropping system Indonesia 24 188 Van Dijk and Bruijnzel 2001 Laurel forest Canary Islands - 6.85 - Aboal et al. 1999 Tropical montane cloud forest Columbia - - 42 Jarvis 2000

2.3. Influence of canopy characteristics on Interception