Are dew measurements relevant for forest litter interception on a Cerrado woodland forest?

<p>Determining the water partitioning in the critical zone, and how the biotic and abiotic factors affect these processes, is crucial to improve the comprehension of hydrological processes. Adequate field measurements of water partitioning in forested areas are challenging. Especially, continuous forest litter interception measurements are difficult to obtain. Therefore, we developed an equipment (named Litter Interception Device - LID), composed of a weighting system that contains a load cell with a resolution of 1 g, for continuous measurements of forest litter interception. The study was carried out in a Cerrado woodland forest (Cerrado sensu stricto) since 2017 in the State of São Paulo, Brazil. Following the continuous monitoring, we observed eventual weight gain during the nights. We analyzed the measurements for possible accumulation of dew in two LIDs between August 2018 and August 2019. We first carried out laboratory tests to check the possibility of measurement errors due to temperature shifts on the load cell. A maximum of 3 g error measurement after 10 °C temperature reduction was observed. We also estimated the dew point temperature for the study area during the monitoring period, based on temperature, relative humidity and rainfall data of sensors installed outside the Cerrado’s forest. In the forest, we monitored the temperature using a thermocouple installed in the forest litter sample. All sensors’ data were stored in a datalogger every 10 min. The dataset was analyzed in daily periods between the 9:00 pm and 7:00 am of the subsequent day. To check for dew accumulation on the forest litter, we defined the following minimum criteria to be considered dew, for each interval of our analysis: (a) the total mass gained could not be less than 2 g (equivalent to 0.0125 mm moisture accumulation); (b) the maximum temperature variation on forest litter 7 °C (considering that daily temperature variations close or above 10 ºC could introduce more errors); (c) there was no rainfall from 9:00 pm to 7:00 am of the subsequent day. On 204 days dew point temperature was reached, from which 76 days at least one of the LIDs registered a weight gain. During the study period, the temperature on the forest litter presented a maximum and mean variation of 6.7 °C and 2.5 (±1.2) °C, respectively. The data analysis indicated on average 4.59 mm of dew in one year. This average corresponded to 0.35% of the total rainfall for the study period (1206 mm) and 3.74% of the total average forest litter interception (133 mm). In tropical forests like the wooded Cerrado presented here, rainfall is the major input of water; otherwise to arid regions, were studies have shown that dew is the major input (i.e. Negev Desert). In our case, despite the low percentage related to the total rainfall, dew should not be neglected. As the LID measures all the mass inputs, including forest litter’s deposition, dew must be considered to correctly determine the hydrological processes at different time-space scales.</p>

Determining forest litter interception in an area of the Cerrado sensu stricto

ABSTRACT Mainly due to the difficulty of directly measuring forest litter interception, many empirical studies on water balance in forests have disregarded this component. In order to assess the magnitude of forest litter interception in an area of the Cerrado s.s. , an estimate of the intercepted volume was made based on equations from two forest litter parameters (Cmax and Cmin) obtained from laboratory assays and monitoring the forest litter quantity. The estimates obtained from the litter interception for 2015 and 2016 were compared with the calculated values of evapotranspiration and internal precipitation of the study area. The total volume of litter interception corresponded on average to 13.4% of the internal precipitation and 8.5% of the total rainfall. Regarding the total annual evapotranspiration, the evaporation from the forest litter interception volume corresponded on average to 10.3% (122 mm) of this total. Forest litter interception for areas such as the Cerrado sensu stricto can have a significant impact on the water balance. This shows the need for directly measuring the forest litter interception, providing more accurate determinations of the rainfall partitioning in these forest areas.

Forest Litter Interception Model for a Sessile Oak Forest

Models that describe hydrological processes in forests may help to estimate the consequences of forestry interventions or of climate change. The authors employed a hydrologic model for estimation of forest litter interception of a middle-aged sessile oak (Quercus petraea) stand. Antecedent water content and the storage capacity of the forest litter were the main parameters of the model. The antecedent water content of the litter was estimated by the daily precipitation and temperature data, collected in Hidegviz Valley research catchment in a three year measurement period (2006-2008). The measurements were done by an instrument we developed ourselves, where the undisturbed forest litter samples were enclosed in frames and measured in daily time steps.

Modelling canopy and litter interception in commercial forest plantations in South Africa using the Variable Storage Gash model and idealised drying curves

There remains a gap in the knowledge of both canopy and litter interception processes in forest hydrology and limitations in the models used to represent them. In South Africa, interception is typically considered to constitute only a small portion of the total evaporation and in some models is disregarded. Interception is a threshold process, as a certain amount of water is required before successive processes can take place. Therefore an error or false assumption introduced in modelling interception will automatically introduce errors in the calibration of subsequent models/processes. Field experiments to assess these processes, viz. canopy and litter interception were established for the three main commercial forestry genera in South Africa, namely Pinus, Acacia and Eucalyptus, which are described in a companion paper. Drawing on both field and laboratory data, the "Variable Storage Gash" model for canopy interception and an idealised drying curve litter interception model were developed to represent these processes for South African conditions. The Variable Storage Gash model was compared with the original Gash model and it was found that it performed better than the original model in forests with high storage capacities yet was similar to the original model in stands with a low storage capacity. Thus, the models developed here were shown to adequately represent the interception processes and provide a way forward for more representative water resources planning modelling. It was found that canopy and litter interception can account for as much as 26.6% and 13.4% of gross precipitation, respectively, and are therefore important hydrological processes to consider in forested catchments in South Africa. Despite the limitation of both the Variable Storage Gash model and the idealised drying curve litter interception model being reliant on empirical relationships, their application highlights the importance of considering canopy and litter interception in water resources management and planning.

Field data collection and analysis of canopy and litter interception in commercial forest plantations in the KwaZulu-Natal Midlands, South Africa

It is well accepted that the total evaporation in forested areas is greater than in grasslands, largely due to the differences in the amount of rainfall that is intercepted by the forest canopy and litter and due to higher transpiration rates. However, interception is the least studied of these components of the hydrological cycle. The study aims to measure and quantify the canopy and litter interception by Eucalyptus grandis, Pinus patula and Acacia mearnsii, at the Two Streams research catchment in the KwaZulu-Natal Midlands of South Africa for the three-year period April 2008 to March 2011. The results from this study showed that canopy and litter interception contributed a significant amount of the water evaporated in a forest water balance. The canopy interception by E. grandis, A. mearnsii and P. patula was 14.9%, 27.7% and 21.4% of gross precipitation, respectively, while litter interception was 8.5%, 6.6% and 12.1% of gross precipitation, respectively.

Modelling canopy and litter interception in commercial forest plantations in South Africa

There is a gap in the knowledge of both canopy and litter interception in South African forest hydrology. Interception is typically considered to constitute only a small portion of the total evaporation and in some models is disregarded. Interception is a threshold process, as a certain amount of water is required before successive processes can take place. Therefore an error introduced in modelling interception, especially disregarding it, will automatically introduce errors in the calibration of subsequent models/processes. Field experiments to assess these processes, viz. canopy and litter interception were established for the three main commercial forestry genera in South Africa, namely, Pinus, Acacia and Eucalyptus. Drawing on both field and laboratory data, the "variable storage Gash" model for canopy interception and an idealised drying curve litter interception model were developed to represent these processes. It was found that canopy and litter interception can account for as much as 26.6% and 13.4% of gross precipitation, respectively, and are therefore important hydrological processes. The models developed were able to adequately represent these interception processes and provide a way forward for more representative water resources planning modelling.

Field data collection and analysis of canopy and litter interception in commercial forest plantations in the KwaZulu-Natal Midlands, South Africa

It is well accepted that the total evaporation in forested areas is greater than in grasslands, largely due to the differences in the amount of rainfall that is intercepted by the forest canopy and litter and higher transpiration rates. However, interception is the least studied of these components of the hydrological cycle. The study aims to measure and quantify the canopy and litter interception by Eucalyptus grandis, Pinus patula and Acacia mearnsii, at the Two Streams research catchment in the KwaZulu-Natal Midlands of South Africa for the three year period April 2008 to March 2011. The results from this study showed that canopy and litter interception contributed a significant amount of the water evaporated in a forest water balance. The canopy interception by E. grandis, A. mearnsii and P. patula was 14.9%, 27.7% and 21.4% of gross precipitation respectively, while litter interception was 8.5%, 6.6% and 12.1% respectively.

New technique to measure forest floor interception – an application in a beech forest in Luxembourg

In hydrological models, evaporation from interception is often disregarded, combined with transpiration, or taken as a fixed percentage of rainfall. In general interception is not considered to be a significant process in rainfall-runoff modelling. However, it appears that on average interception can amount to 20–50% of the precipitation. Therefore, knowledge about the process of interception is important. Traditional research on interception mainly focuses on canopy interception and almost completely denies forest floor interception, although this is an important mechanism that precedes infiltration or runoff. Forest floor interception consists partly of interception by dry soil, partly of interception by short vegetation (mosses, grasses and creeping vegetation) and partly of interception by litter. This research project concentrates on litter interception: to measure its quantities at point scale and subsequently to upscale it to that of a hydrotope. A special measuring device has been developed, which consists of a permeable upper basin filled with forest floor, and a watertight lower basin. Both are weighed continuously. The device has been tested in the Huewelerbach catchment (Luxembourg). The preliminary measuring results show that the device is working properly. For November 2004, evaporation from interception was calculated to be 14 mm of 42 mm throughfall (i.e., 34%).