A Reconnaissance Study of Water and Carbon Fluxes in Tropical Watersheds of Peninsular Malaysia: Stable Isotope Constraints

Abstract

Evapotranspiration is a nexus for planetary energy and carbon cycles, as yet poorly constrained. Here I use stable isotopes of oxygen and hydrogen to partition flux of water due to plant transpiration from the direct evaporative flux from soils, water bodies and plant. The study areas, Langat and Kelantan watersheds represent examples of domains dominated by the respective Southwest and Northeast monsoons on the two sides of the main orographic barrier (Titiwangsa mountain range). Mean annual rainfall for the Langat watershed, obtained from 30 years of hydrological data, is 2145 ± 237 mm. Tentatively, 48% of this precipitation returns to the atmosphere via transpiration (T), with 33% partitioned into discharge (Q), 8% into interception (In), and 11% into evaporation (Ed). In the Kelantan watershed, the mean annual rainfall, also based on the 30 year hydrological data, is 2383 ± 120 mm. Similar to Langat, the T accounts for 43% of precipitation (P), 45% is discharged into South China Sea (Q), 12% partitioned into interception (In) and tentatively 0% for evaporation (Ed). Ed for the Langat watershed represents only a small proportion in terms of volumetric significance, up to almost ~11% with strong effect on the isotopic fingerprints of waters associated with the summer Southwest Monsoon (SWM). Note, however, that insignificant Ed for the Kelantan watershed may be an artefact of rain and river water sampling at only coastal downstream portion of the watershed. High humidity (80%) also was recorded for the Malaysian Peninsula watershed.

T appropriates about half of all solar energy absorbed by the continents, here ~1000*103 g H2O m-2 yr-1 similar to other tropical regions at 900-1200*103 g H2O m-2 yr-1. The associated carbon fluxes are ~ 1300 g C m-2yr-1, independent of P. Vegetation responses to solar irradiance, via T and photosynthesis reflects the importance of stomatal regulation of the water and carbon fluxes. In order to maintain high transpiration in the tropical region, “constant” water supply is required for continuous pumping of water that delivers nutrients to the plant, suggesting that water and carbon cycle are co-driven by the energy of the sun. The existence of the water conveyor belt may be precondition for nutrient delivery, hence operation of the carbon cycle. Potentially, this may change our perspective on the role that biology plays in the water cycle. In such perspective, the global water cycle is the medium that redistributes the incoming solar energy across the planet, and the anatomical structures of plants then help to optimize the loop of energy transfer via evaporation and precipitation in the hydrologic cycle.

The main features of aquatic geochemistry of the Langat and Kelantan rivers inferred from the Principal Component Analysis are controlled by three components that explain 80% and 82% of total variances. These components are reflecting of the geogenic factor with superimposed pollution, the latter particularly pronounced in urbanized sections of the Langat river and dominant in downstream of the Kelantan river. There is no correlation between seasonal variations in major ion chemistry and environmental variables such as precipitation, discharge, temperature or solar activity.