Free Throughfall Coefficient Research Articles

The performance of the reformulated Gash rainfall interception model in the Hyrcanian temperate forests of northern Iran

• Reformulated Gash Analytical models (RGAM) applied for seven reforestations. • Models need frequent numbers of rainfall to estimate rainfall interception loss. • RGAM is advisable for predicting interception with small numbers of rain storms. • Ecohydrological parameters dramatically differs between studied stands. Modeling rainfall interception ( I ) is fundamental for understanding the forests’ role in ecohydrological processes in local and regional scales. This research examines the Reformulated Gash Analytical Model (RGAM) applicability for estimating I in seven dissimilar plantations ( Acer velutinum , Quercus castaneifolia , Pinus brutia , Picea abies , and Cupressus sempervirens L. var horizontalis ) in the four reforested areas across the Hyrcanian temperate forests, northern Iran. Field measurements were performed from May-2012 to March-2014 for 167 distinct throughfall and gross rainfall observations. According to our results, the effects of inter-species variation on I value was evident, as P. abies might be the most excellent among the five frequently used tree species for planting project in temperate forests if the primary goal of plantation is to decrease erosion and surface runoff as it intercepted and evaporated more rainfall than other species. Our findings showed that in terms of average-based calculation, the mean of free throughfall coefficient ( p ) value in evergreen needle-leaved reforestations (0.47) was lower than those calculated for deciduous broad-leaved reforestations (0.59), and vice versa for the canopy storage capacity ( S ; 1.85 mm vs . 1.36 mm, respectively), and mean of the ratio of evaporation rate to the mean of rainfall intensity during saturated canopy conditions ( E ¯ / R ¯ ; 0.45 vs . 0.38, respectively). The RGAM satisfactorily estimated I for all species (except for A. velutinum stand), because the estimation error of I was lower than 15% and the Nash–Sutcliffe efficiency was equal to or higher than 0.87. In conclusion, using data from a short time study period to derive canopy ecohydrological parameters as well as modeling I by the RGAM is advisable in reforestation areas across the Hyrcanian temperate forests and regions with similar forest and meteorological attributes.

Long-term investigation of forest canopy rainfall interception for a spruce stand

The factors influencing the interception process such as the vegetation structure and the meteorological conditions, especially rainfall characteristics, are diverse and highly variable. Therefore, derivation of canopy parameters for interception modelling requires data from field experiments over a long period. In this study, we analyse long-term changes in canopy parameters and rainfall interception for an old-growth spruce stand (composed mainly of Picea abies at a continuous flux site started within EUROFLUX in 1996) within the Tharandter Wald, Southwest of Dresden, Germany. The 10-minute data from the continuous measurements of gross precipitation (P) and throughfall (TF) from 1997 to 2018 were used as follows: Rainfall events were separated by a minimum time of three hours without rainfall and divided into rainfall classes according to their total amount and intensity. Due to vegetation changes in the measuring period caused by two thinning operations, the complete study period was divided into three periods. Canopy parameters S (storage capacity), p (free throughfall coefficient) and Ps (canopy saturation point) were then generated applying a regression-based method using the relationship between P and TF. We generated parameter sets for the complete period, for the three periods reflecting the vegetation changes and for single years. These parameter sets were further tested using a regression model.The statistical analysis of the long-term data set showed that stand parameters and interception are largely influenced by vegetation changes. S and Ps decreased with each thinning. Interestingly, interception and rainfall only changed after the first thinning (increase in p, decrease in interception), indicating counter-acting factors after the second thinning, such as changes in meteorological conditions, rainfall patterns or thinning induced changes in the microclimate of the canopy.Generally, the reliability of the derived canopy parameters depends on the length of the dataset used for the regression analysis. However, the parameter sets that considered three periods produced the best model fit compared to parameter sets for the complete period or for single years. This clearly shows that model performance of statistical interception models depend on a reliable parameterisation that can be improved when taking into account changes in stand characteristics.

Importance of transitional leaf states in canopy rainfall partitioning dynamics

The temporal dynamics of forest canopy rainfall partitioning are important to forest ecology and management as it influences all subsequent hydrological processes along the rainfall-to-discharge flow path. Despite a growing body of literature on the importance of coupled hydrological–ecological interactions during periodic forest life cycle events, little work has examined how canopy rainfall partitioning varies across transitional leaf states (between the leafed vs. leafless states). This study analyzed a 3 year field monitoring campaign for two tree species in semiarid Iran (Robinia pseudoacacia and Platanus orientalis) to describe rainfall partitioning dynamics across the full-leaf, senescence, leafless, and leafing states. Crown saturation point, canopy storage capacity, free throughfall coefficient and the ratio of wet canopy evaporation rate to mean rainfall intensity were related to decreases/increases in plant area index and canopy closure. The high variability of rainfall partitioning observed in this study highlights the importance of transitional leaf states in the temporal characterization of water inputs to forest surfaces and boundary layer.

A seasonal evaluation of the reformulated Gash interception model for semi-arid deciduous oak forest stands

Abstract Interception represents a significant component of the water budget of forests, diverting significant quantities of precipitation away from soil moisture, transpiration, and surface and groundwater recharge. There are several physically-based models of forest interception but their utility across seasonal variability is limited because few studies have collected field data of both the growing (leaf on) and dormant (leaf off) seasons and explored how seasonality affects parameters of simulations. Rainfall partitioning modelling using the Reformulated Gash Analytical Model (RGAM) for Brant’s oak (Quercus brantii) forest plots in the Zagros forest, Iran, simulated interception values close to the observed, with an underestimation of 4.0% and 7.5% for the leafed and leafless periods, respectively. Model parameters varied seasonally: free throughfall coefficient, canopy storage capacity, canopy saturation point, and ratio of mean wet evaporation rate to mean rainfall intensity were ∼0.60, 1.0 mm, 2.6 mm, and 0.15 in the leafed period and ∼0.80, 0.1 mm, 0.5 mm, and 0.04 in the leafless period, respectively. In this application, the RGAM was highly sensitive to change in the ratio of mean wet evaporation rate to mean rainfall intensity, and less sensitive to canopy cover and canopy storage capacity, especially in the leafless period. Therefore, canopy structure is less important for rainfall interception predictions by RGAM.

Oak trees decline; a sign of climate variability impacts in the west of Iran

The Persian oak, Quercus brantii, trees in the Zagros region of Western Iran have been in decline since 2000. The decline is assumed to be highly connected with changes in meteorological parameters. Our objectives were to quantify the long-term trends in meteorological parameters and reference evapotranspiration (ET0) in the Zagros region and estimate ecohydrological parameters highly affected by climate variability and related to rainfall interception (I) process (i.e., canopy storage capacity (S), the ratio of mean evaporation rate from the wet canopy to the mean rainfall intensity (E/R), and the free throughfall coefficient (p)). Long-term (1961-2010) changes in air temperature (T), precipitation (P), and wind speed (WS) were obtained from six synoptic meteorological stations located in the region. Throughfall (TF) was measured using the sixteen rain gauges randomly located underneath the crown of the five individual trees. P was measured using rain gauges fixed in an open space nearby to the oak trees .I was computed as the difference between P and TF. From 2000 to 2010, meteorological parameters and ET0 changed slightly; T, WS, and ET0 increased (+0.6 ˚C, +0.4 m.s-1, +0.25 mm.day-1, respectively), while P decreased (-60 mm).When climate patterns between 1961-2010 were analyzed for 6 synoptic weather stations, P significantly decreased significantly at one station, whereas T significantly increased at two stations and significantly decreased at another. I was estimated to be 40% and 25% within in-leaf and leaf-less periods, respectively. During the in-leaf period, the mean values of S, E/R, and p were roughly estimated to be 1 mm, 0.22, and 0.23, respectively. Our results indicate that the Zagros region is getting warmer and oak trees will indisputably experience reduction in the available water because of increased evaporative loss.

Rainfall Interception in a Robinia pseudoacacia Forest Stand: Estimates Using Gash’s Analytical Model

The authors have studied the principal components of rainfall interception loss in a planted forest stand of Robinia pseudoacacia on the Loess Plateau. The purpose was to provide new information about the applicability of the original Gash analytical model to a new geographic location and to one of the primary species being used in the region’s reforestation program. The authors estimated forest structure parameters, including the mean evaporation rate, canopy storage capacity at saturation, free throughfall coefficient, rainfall fraction diverted to the trunks, and trunk storage capacity by using the intercepts and slopes obtained from regression analyses of the measured interception loss, throughfall, and stemflow versus gross rainfall. The interception and components of interception loss for trees in a Robinia pseudoacacia forest located on a south-facing slope were calculated using Gash’s analytical model. The total estimated interception loss during the period of observation was 10.8% greater than that calculated on the basis of measurements of the gross rainfall, throughfall, and stemflow. The good agreement between the estimated and measured values indicates that Gash’s analytical model is suitable for estimating interception loss in forests on the Loess Plateau of China.

Evaporation and storage of intercepted rain analysed by comparing two models applied to a boreal forest

Rainfall and throughfall were measured during the summer of 1995. Rainfall interception is often simulated by a version of the well-known Rutter-Gash analytical model. In this study this model was compared to a model based on an exponential saturation equation. The concept of the ‘minimum method’ for deriving canopy storage capacity and free throughfall coefficient by the Leyton-analysis, is compared to the concept of maximum storage capacity by reversing the models. Measured evaporation rate during rain events was found to be lower than simulated by the Penman equation using different known formulations for aerodynamic resistance. The concept of a high internal canopy resistance and decoupling of the canopy from the atmosphere should be analysed further in order to explain low evaporation during rainfall.

Modelling of rainfall interception loss in agroforestry systems

Two models of interception loss have been tested against new field data obtained in widely-spaced stands of Sitka spruce trees. The Gash model and a modified version of the Rutter model, have been used with data from an automatic weather station, to predict interception loss using parameters obtained from observations made in 1988 and 1989. The predictions for an eight-week period during 1987 were compared with measurements of interception loss. Good agreement between observed and predicted interception loss was obtained with both models over the whole period. The modified Rutter model gave better predictions than the Gash model for individual storm events and performed better at the wider spacings. The sensitivity of both models to the major characteristics of the tree stand structure in agroforestry systems was also investigated and it was shown that interception loss was most sensitive to boundary layer conductance and free throughfall coefficient.

Rainfall interception by bracken in open habitats — Relations between leaf area, canopy storage and drainage rate

A rainfall simulator has been used to investigate how the free throughfall coefficient p, canopy storage C, and drainage rate D s, of bracken varies with projected leaf area index ( LAI) over a LAI range of 0.4 to 5.88. For field canopies p was found to be a simple exponential function of LAI. Measured maximum water shortage, C max was related to LAI by C max = 0.467 (± 0.004) LAI. Attempts to relate the measured storage and drainage rate using the modified Rutter drainage function and the recently proposed Calder drainage model were unsuccessful, primarily because both assume zero drainage at zero C. The experimental data show that C always has some positive value C min when drainage from the canopy ceases. C min was related to LAI by C min = (0.156 ± 0.004) LAI, and is smaller than reported values ∗ ∗ Leyton et al. (1967). Two new asymptotic drainage functions were fitted to the data, using optimisation with excellent results: D s = [e K( C− C min)] − 1 (I) D s = A( C − C min) K (II) optimisation of eqns. (I) and (II) for each experiment showed that the values C min, K and A were simple functions of LAI. Thus both empirical expressions could be expressed as simple functions of LAI and storage, and hence generalized over the complete LAI range. Model (I) explained over 90% of the variance of D s over the LAI range 0.4 to 5.88. If remote sensing techniques are used to obtain values of LAI, the functions presented have wide applicability to bracken growing in open habitats.

Estimation of Canopy Reservoir Capacity and Oxygen-18 Fractionation in Throughfall in a Pine Forest

Amount and oxygen-18 content of rainfall and throughfall have been monitored in a pine forest in Uppsala during summer 1983. It is found that the free throughfall coefficient, C, the canopy reservoir capacity, S, and the critical value of rain, R0, are ≈ 0.5, 2.1 mm and 4.1 mm respectively. The average value of throughfall is about 60% of rainfall. Generally, the throughfall is enriched in oxygen-18 with respect to rainfall, but in very few cases depletion has been observed. The enrichment or depletion in throughfall have been graphically predicted and agree fairly well with the observations. Oxygen-18 values of throughfall have been simulated by a model describing isotopic fractionation in throughfall. Using estimated relative humidity and temperature during the rainfall, the simulations describe quite closely the observed fractionation. It is felt that more accurate estimates of C, S and fractionation in throughfall can be obtained by monitoring rain and throughfall in small fractions of about 0.2 mm or even less.

Further interception studies of heather using a wet-surface weighing lysimeter system

Measurements have been made of the four canopy parameters required by Rutter type interception models for heather ( Calluna vulgaris). The values obtained are 5.13 and 0.00085 for the drainage parameters b and k, respectively, (0.13 ± 0.09) for the free throughfall coefficient p, and values of 1.1 ± 0.1 and 0.85 ± 0.1 mm for the canopy capacity S, derived from independent drainage and evaporation experiments, respectively. It is suggested that the difference between the two values of S is significant and represents a real difference between the canopy storage at which drainage ceases and the smaller canopy storage necessary for a completely wet canopy. Studies of the wetting behaviour of heather provide evidence to support this hypothesis.

RAINFALL INTERCEPTION IN A FOREST IN THE VELEN HYDROLOGICAL REPRESENTATIVE BASIN

In the summer and autumn of 1973, 25 rain troughs were operating below canopy and four in clearings of a predominantly coniferous forest in the Velen hydrological research area. The collected rain water amounts were measured 14 times (about once a week). Using these data and the records from a tipping-bucket rain gauge in a simple model for throughfall, it was found that the overall water storage capacity of the forest canopy was about 2 mm and that the free throughfall coefficient p was about 0.5. Results from 50 photographs taken upward from the troughs indicate a smaller p-value. The time for evaporation of the intercepted water after an average rain storm is roughly estimated as 5 h. About 26 % of the total rainfall was intercepted and evaporated.