A comparative analysis of the flooding and fire effects on the energy exchange in a wetland community (Morichal) of the Orinoco Llanos

Abstract

This work analyzes the effects of flooding and fire stress on a wetland energy balance during the dry season of the Orinoco lowlands. The effect of flooding for burning conditions is evaluated by comparing flux measurements in a flooded/burned (T1) and an unflooded/burned (T3) stand, and the effect of burning for flooding conditions by distinguishing between a flooded/burned (T1) and a flooded/unburned (T2) stand. These examined factors affect the canopy features related to coverage and aerodynamic forcing conditions. The effect of flooding on the average daily radiative budget for the burned stands is to decrease the average daily albedo from 0.14 for the unflooded stand (T3) to 0.08 for the flooded stand (T1). Whereas, the effect of burning for the flooded stands decreases the average daily albedo from 0.16 for the unburned stand (T2) to 0.08 in the burned stand (T1). Furthermore, changes in surface temperature determine variations in the average daily output of long-wave ratios between stands with the most average daily net loss of long-wave radiation (−36.3 MJ m −2 day −1) corresponding to the flooded/burned stand (T1). In the stands, the changes in albedo and surface temperature lead to variation in the available energy. Thus, the average daily net radiation ( R n) is affected by burning of the flooded stands, being 15.2 MJ m −2 day −1 in T1 and 14.7 MJ m −2 day −1 in T2. The effect of flooding on the average daily R n is evident when compared T1 to T3 (13.1 MJ m −2 day −1). The partitioning of R n into sensible ( H) and latent (LE) heat flux over the three stands is measured simultaneously using the Bowen ratio/gradient method, which is temporally corroborated by means of the eddy covariance approach. Results indicate that average daily LE to average daily R n ratio is 0.78, 0.78 and 0.80 for T1, T2 and T3, respectively. However, the partitioning of average daily LE into transpiration and evaporation is affected clearly by the analyzed factors. Thus, burning reduces the transpiration rate from −8.5 to −3.3 MJ m −2 day −1 for the flooded stands (T2 and T1, respectively). Whereas, flooding varies only a small amount the average daily transpiration rate in the burned stands (T1 and T2) (−3.3 and −3.4 MJ m −2 day −1). Over the stands, the average daily H in relation to R n for T1, T2 and T3 was 0.19, 0.21 and 0.22, respectively. In relation to the net storage heat flux (ΔSH), the water table at or above the sediments is a small heat sink as the radiation intercepted by the canopies is relatively large. The average daily net storage heat flux is enhanced slightly in the flooded stand (T1) as compared to that for the unflooded/burned stand (T3). Whereas, in the surface of the unburned stand, ΔSH is in thermical equilibrium. Evaluation of evapotranspiration models as a function of the of hourly LE measured over the three stands, indicates that the Penman evapotranspiration model for equilibrium is appropriated for modeling long-term evapotranspiration, and therefore LE is controlled mainly by the available energy and temperature. However, the slope is above 1 (1.11, 1.12 and 1.04 for T1, T2 and T3, respectively). This possible surface control on the results is corroborated by the Penman–Monteith and Slatyer-McIlroy model for a negligible surface resistance. The modeled values are over-predicted and consequently surface resistance affects partially the results. The effect of the areal surface resistance on wetland potential evapotranspiration is assessed as the wetland is surrounded by a dry savanna environment. The relationship between wetland potential evaporation and savanna evaporation is evaluated using the Lhomme closed-box model of the convective boundary layer. The results indicate that the savanna surface resistance enhances the wetland potential evaporation at equilibrium. The results reported in this analysis are pertinent to similar Orinoco wetlands, nevertheless the controls and functioning of herbaceous wetlands under the described land-use practices are revealing common features for wetlands surrounded by regional savannas in South America.