Deposition

3.3.1 Water balance

The water mass balance was used in several studies to estimate fog water inputs (Asbury et al. 1994; Bruijnzeel 2001). The aim of the FIESTA project is to model the impact of defor-estation on the waterbudget. The knowledge of the correct water input to the ecosystem - by fog and rain - is essential. Hence the water balance was applied to control whether the water input to the forest was measured correctly. Following formula was applied:

P +F =T F +SF + ∆CS+E_i , (3.1)

where P is the deposition of rain, F the fog water deposition caused by turbulence and gravitational settling,T F is throughfall,SF is stemflow and∆CSis the change in canopy storage. E_i is the evaporation from a wet canopy, calculated with the Penman-Monteith equation using data from thermocouples, net radiation, and windspeed. It was assumed that the canopy was wet when there was a signal either from a verical or a horizontal raingauge. In this campaign, the values for a 65 days period, between day 68 and 133, are: Precipitation: 406.3 mm, Fog: 16.6 mm, T F: 521.6, SF (2% of T F): 10.4 mm,Ei: 17.1 mm. ∆CS was neglected. At a first try, the balance does not come out even: the un-explained amount is 126.2 mm. In this study, a sophisticated set-up was used to measure fog water fluxes directly above the cloud forest canopy. For this reason, we expect the fog water inputs to be correct. The deviation from the balance is more likely to be generated by incorrect rainfall measurements. There were many rain events observed with small droplet sizes and high windspeeds, generating horizontal rain. Under these conditions, an under-estimation of rainfall amounts is probable (Sharon 1980). On account of this, we tried to correct the rain amounts in order to close the water budget.

3.3.2 Correction of Precipitation Measurements

Sharon (1980) described the influences of rain angle, storm direction, slope inclination, and aspect of slope on the difference between conventionally measured and effective hy-drological rainfall. It was found that on a windward facing slope hyhy-drological rainfall can exceed conventional gauge measurements by more than 100%, depending on slope and rainfall inclination. To correct the rainfall measurements of this campaign, the following

15 formula was used (Sharon 1980):

Pa=P0[1 + tan(a)·tan(b)·cos(za−zb)] (3.2) whereP_ais the effective hydrological rainfall,P₀ the conventionally measured,athe angle of the slope, b the rainfall inclination,z_a the aspect of the slope andz_b the rain direction.

The rain direction was given by the wind direction and the angle of rainfall was calculated as follows (Herwitz and Slye 1995):

tan(b) = W/U_v , (3.3)

wherebis the angle of rainfall in degrees from the vertical, W is the horizontal wind speed (m s⁻¹) andU_v is the terminal fall velocity (m s⁻¹). Terminal fall velocity was calculated after Herwitz and Slye (1995):

U_v = 3.378·ln(D) + 4.213 , (3.4)

whereDis the raindrop diameter (mm). Raindrop diameter was computed on the basis of rainfall intensity (Herwitz and Slye 1995):

D= 2.23·(0.03937P)^0.102 , (3.5) whereP is the rainfall intensity (mm h⁻¹). Rainfall angles for the period between day 68 and 133 were between 1.43 and 66.27 degrees from the vertical, with a mean of 31.30 degrees. Assuming a slope inclination of 30 degrees and a slope aspect of 20 degrees from north, the corrected rainfall amount for the period between day 68 and 133 is 510.1 mm.

This is 25.5% more than the conventionally measured amount. The largest corrections are made for events with high windspeeds (Figure 3.4).

3.3.3 Water Balance with Corrected Rainfall Amounts

With the corrected amount of rainfall, the water balance (entire water amount during the time period on which the correction was applied) looks as follows:

510.1_P + 16.61_F <521.56_{T F} + 10.42_SF + 17.09_Ei (3.6) The missing amount of water is 22.36 mm. The temporal distribution of the deviation shows that there is still a discrepancy from the optimal balance (see Figure 3.4).

Apparently, the largest deviation originates during the event between day 90 and 94.

This event excluded, the balance is even negative, i.e. there was too much rainfall or, more likely, the water was intercepted by epiphytes and evaporated before reaching the ground; it was assumed that∆CScan be neglected, but if there is a dry up of the canopy, smaller throughfall than rain amounts could be possible. Another possibility is an error created by the measuring setup: throughfall was measured at the opposite slope with an aspect of 340 degrees from north. Arazi et al. (1997) state in their study that small scale topographical inhomogeneities substantially influence the rainfall distribution. Therefore,

16

day

corrected rain amount (mm) deviation from balance (mm) optimal balance

69 72 75 78 81 84 87 90 93 96 99 102 106 110 114 118 122 126 130

−20020406080100120140

Water balance with corrected rain amount

−20020406080100120140

Windspeed

day

m s⁻¹

69 72 75 78 81 84 87 90 93 96 99 102 106 110 114 118 122 126 130

024681012

Figure 3.4: Corrected daily rainfall amounts after the application of Sharon’ modell (upper graph) and their deviation from the closed water balance. Lower panel: daily mean windspeed.

17 an uneven rainfall distribution in the catchment could be an explanation for the smaller rain amounts in the area where throughfall was measured. But still, there is an error in the water balance during event of days 90–94. Is the correction of the rain amount appropriate for events with wind speeds above 10 m s⁻¹? Was the rain angle calculated wrongly? With a rain angle of 70 instead of 60 degrees the correction of the rain amount would yield a rain amount appropriate to the throughfall amount. Or was the rain amount underestimated as a result of the tipping bucket system, which is less accurate in case of high rainfall intensities? Another possibility is a malfunctioning of the measurement setup because of the storm event.