Reply to ‘Comment on: Rainfall,fog and throughfall dynamics in a subtropical ridge top cloud forest,National Park of Garajonay (La Gomera,Canary Islands,Spain)’ by A. Ritter and C.M. Regalado

This Reply aims to clarify and address many of the errors introduced to the discussion of our original paper (García‐Santos and Bruijnzeel, 2011; referred to henceforth as GSB2011) in the comment by Ritter and Regalado (2012) (henceforth referred to as RRR2012) as explained more fully under replies nos. 1–6, 8–12 and 14–18 below. Two (largely inconsequential) shortcomings in our original paper as detected by RRR2012 are acknowledged (see replies nos. 7 and 13 below). It is concluded that the majority of the comments offered by RRR2012 are unfounded as well as unnecessary. In summary, the two errors in GSB2011 spotted by RRR2012 (see points 7 and 13 below) could easily have been addressed in an erratum. Copyright © 2013 John Wiley & Sons, Ltd.     

Sugar cane nutrient requirements and the role of atmospheric deposition supplying supplementary fertilization in a Venezuelan sugar cane plantation

As a consequence of high industrial and agricultural activities, acidic rains loaded with pollutants—including nutrients—are characteristic of northern central Venezuela, a region dominated by sugar cane plantations. Canopies of forest and agricultural crops can modify the chemistry of rainfall through uptake, leaching and outwash of deposited ions. This paper describes the change in the chemistry of acid rains after passing through a sugar cane canopy. Four plots of 300 m² within a 4.5 ha experimental area, planted with Saccharum officinarum had rain and throughfall collectors installed. The study corresponds to the analysis of the growing season of the third ratoon. The pH of the rain in the agroecosy stem increased after passing through the canopy. The magnitudes of the changes were important and partially related to the significant amount of cations leached from the leaves or washed out from dry deposition to the leaves. N inputs for wet and dry deposition in the agroecosystem were high (25.25 kg ha^?1 yr^?1) as a consequence of the agricultural activity in the area, the local burning of sugar cane before cropping, and the location of the experimental area close to petrochemical and fertilizer industries. Rainfall constitutes an important source of nutrient inputs to the sugar cane system. In the case of macronutrients (N, P and K) the inputs were high and supply an important fraction of plant nutrient needs, as occurs for zinc and copper.     

Throughfall and its spatial variability in a temperate rainforest of simple structure

There are few throughfall data from southern hemisphere closed-forest, and none from Tasmanian callidendrous cool-temperate rainforest, which has a simpler structure than most primary rainforests. We determined throughfall, measured its local spatial variation, and tested its relationships with rainfall, rainfall intensity, wind speed, canopy dryness, canopy cover, and other structural variables in a cool-temperate callidendrous rainforest in Tasmania. Eighty-two percent of the precipitation was measured as throughfall, which occurred after 2.3 mm of rain fell on dry canopies. The cumulative rainfall in 25 randomly located funnel rain gauges on the forest floor varied from 160 to 567 mm. Canopy cover and other structural variables did not predict the spatial pattern of throughfall. While throughfall in rainfall events was related to rainfall amount and intensity, wind speed did not affect throughfall as a percentage of rainfall. Percentages of throughfall to rainfall over 100 for many low rainfall events may indicate a contribution of fog drip to precipitation on the forest floor. The high local spatial variability in throughfall indicates the mean moisture conditions on the forest floor may not be a good indicator of the potential for localised fire damage.     

Shallow groundwater response to rainfall on a forested headwater catchment in northern coastal California: implications of topography,rainfall, and throughfall intensities on peak pressure head generation

The pore water pressure head that builds in the soil during storms is a critical factor for the prediction of potential slope instability. We report findings from a 3‐year study of pressure head in 83 piezometers distributed within a 13‐ha forested catchment on the northern coast of California. The study's primary objective was to observe the seasonal and storm‐based dynamics of pressure head at a catchment scale in relation to observed rainfall characteristics and in situ topography to better understand landscape patterns of pressure head. An additional goal was to determine the influence of the interaction between rainfall and forest canopy in altering delivery of water and pressure head during the large storms necessary to induce landsliding. We found that pressure head was highly variable in space and time at the catchment scale. Pore pressures peaked close to maximum rainfall intensity during the largest storms measured. The difference between rainfall and throughfall delivered through the canopy was negligible during the critical landslide‐producing peak rainfall periods. Pore pressure was spatially variable within the catchment and did not strongly correlate with surficial topographic features. Only 23% of the piezometers located in a variety of slope positions were found to be highly responsive to rainfall. Topographic index statistically explained peak pressure head at responsive locations during common storms, but not during the larger storms with potential to produce landslides. Drainage efficiency throughout the catchment increased significantly in storms exceeding 2 to 7 months peak pressure head return period indicated by slowing or cessation of the rate of increase of pressure head with increasing storm magnitude. This asymptotic piezometric pattern persisted through the largest storm measured during the study. Faster soil drainage suppressed pressure head response in larger storms with important process implications for pore pressure development and landslide hazard modelling. Copyright © 2012 John Wiley & Sons, Ltd.     

Cloud water interception in the high altitude tree heath forest (Erica arborea L.) of Paul da Serra Massif (Madeira,Portugal)

Cloud water interception (CWI) occurs when cloud droplets are blown against the forest canopy, where they are retained on the vegetation surface, forming larger water droplets that drip into the forest floor. CWI was measured from 1 October 1997 to 30 September 1999, on a first‐line tree heath (Erica arborea), at Bica da Cana, Madeira Island. Rainfall was corrected for wind‐loss effect and compared with throughfall and other climatological normals. The CWI depletion rate along a forest stand transect was also analysed during three distinct fog events in 2008. Cloud water was 28 mm day^?1, corresponding to 68% of total throughfall and 190% of the gross precipitation. Cloud water correlates directly with monthly normals of fog days and wind speed and correlates inversely with the monthly air temperature normal. CWI has an exponential correlation with monthly relative humidity normal. Cloud water capture depletion along the stand shows a logarithmic decrease. Although a forest stand does not directly relate to a first‐line tree heath, this study shows that CWI is a frequent phenomenon in the Paul da Serra massif. Restoration and protection of high altitude ecosystems in Madeira should be a priority, not only for biodiversity, ecological and economical purposes but also for its role in regional water resources. Copyright © 2011 John Wiley & Sons, Ltd.     

Distribution of throughfall and stemflow in multi‐strata agroforestry,perennial monoculture,fallow and primary forest in central Amazonia,Brazil

The partitioning of rain water into throughfall, stemflow and interception loss when passing through plant canopies depends on properties of the respective plant species, such as leaf area and branch angles. In heterogeneous vegetation, such as tropical forest or polycultural systems, the presence of different plant species may consequently result in a mosaic of situations with respect to quantity and quality of water inputs into the soil. As these processes influence not only the water availability for the plants, but also water infiltration and nutrient leaching, the understanding of plant effects on the repartitioning of rain water may help in the optimization of land use systems and management practices. We measured throughfall and stemflow in a perennial polyculture (multi‐strata agroforestry), monocultures of peach palm (Bactris gasipaes) for fruit and for palmito, a monoculture of cupuaçu (Theobroma grandiflorum), spontaneous fallow and primary forest during one year in central Amazonia, Brazil. The effect on rain water partitioning was measured separately for four useful tree species in the polyculture and for two tree species in the primary forest. Throughfall at two stem distances, and stemflow, differed significantly between tree species, resulting in pronounced spatial patterns of water input into the soil in the polyculture system. For two tree species, peach palm for fruit (Bactris gasipaes) and Brazil nut trees (Bertholletia excelsa), the water input into the soil near the stem was significantly higher than the open‐area rainfall. This could lead to increased nutrient leaching when fertilizer is applied close to the stem of these trees. In the primary forest, such spatial patterns could also be detected, with significantly higher water input near a palm (Oenocarpus bacaba) than near a dicotyledonous tree species (Eschweilera sp.). Interception losses were 6·4% in the polyculture, 13·9 and 12·3% in the peach palm monocultures for fruit and for palmito, respectively, 0·5% in the cupuaçu monoculture and 3·1% in the fallow. With more than 20% of the open‐area rainfall, the highest stemflow contributions to the water input into the soil were measured in the palm monocultures and in the fallow. Copyright © 1999 John Wiley & Sons, Ltd.     

Precipitation interception in Australian tropical rainforests: I. Measurement of stemflow,throughfall and cloud interception

Methods for measuring throughfall, stemflow and, hence, interception in the tropical rainforests of the Wet Tropics region of North Queensland, Australia, were tested at three sites for between 581 and 787 days. The throughfall system design was based on long troughs mounted beneath the canopy and worked successfully under a range of rainfall conditions. Comparison of replicated systems demonstrated that the methodology is capable of capturing the variability in throughfall exhibited beneath our tropical rainforest canopies. Similarly, the stemflow system design which used spiral collars attached to sample trees worked well under a range of rainfall conditions and also produced similar estimates of stemflow in replicated systems. Higher altitude rainforests (>1000 m) in North Queensland can receive significant extra inputs of water as the canopy intercepts passing cloud droplets. This additional source of water is referred to as ‘cloud interception’ and an instrument for detecting this is described. The results obtained from this gauge are compared with cloud interception estimates made using a canopy water balance method. This method is based on stemflow and throughfall measurements and provides an alternative means to fog or cloud interception gauge calibration techniques used in the literature. Copyright © 2007 John Wiley & Sons, Ltd.     

Determination of Sulfur Balance Between the Atmosphere and a Norway Spruce Forest Ecosystem: Comparison of Gradient Dry + Wet and Throughfall Deposition Measurements

Total sulfur deposition was determined above a Norway spruce forest, in Hungary. Two methods were applied, on one hand dry + wet deposition measurements and on the other, throughfall and stemflow deposition estimations have been carried out. Results show: total depositions are 3.3 and 3.2 g S m^–2 yr^–1 determined by dry + wet and throughfall deposition methods, respectively. The share of the dry deposition in the total S-load is 73%. The agreement between the results of the two different methods is good and suggests the needlessness of complicated dry + wet flux measurements, i.e. the total and dry deposition of sulfur compounds to forests can be determined by simple throughfall and wet deposition measurements.