Parsimonious modeling of vegetation dynamics for ecohydrologic studies of water‐limited ecosystems

Abstract

[1] The structure and function of vegetation regulate fluxes across the biosphereatmosphere interface with large effects in water-limited ecosystems. Vegetation dynamics are often neglected in hydrological modeling except for simple prescriptions of seasonal phenology. However, changes in vegetation densities, influencing the partitioning of incoming solar energy into sensible and latent heat fluxes, can result in long-term changes in both local and global climates with resulting feedbacks on vegetation growth. This paper seeks a simple vegetation dynamics model (VDM) for simulation of the leaf area index (LAI) dynamics in hydrologic models. Five variants of a VDM are employed, with a range of model complexities. The VDMs are coupled to a land surface model (LSM), with the VDM providing the LAI evolution through time and the LSM using this to compute the land surface fluxes and update the soil water contents. We explore the models through case studies of water-limited grass fields in California (United States) and North Carolina (United States). Results show that a simple VDM, simulating only the living aboveground green biomass (i.e., with low parameterization), is able to accurately simulate the LAI. Results also highlight the importance of including the VDM in the LSM when studying the climate-soil-vegetation interactions over moderate to long timescales. The inclusion of the VDM in the LSM is demonstrated to be essential for assessing the impact of interannual rainfall variability on the water budget of a water limited region.

Soil moisture and vegetation controls on evapotranspiration in a heterogeneous Mediterranean ecosystem on Sardinia, Italy

“…The grass b function (b g ) suggests a wilting point of q $ 0.1, and a commencement of transpiration reduction below about 0.2. Note that for high q values the b g doesn't reach a value of 1 because other environmental factors (e.g., temperature, vapor pressure deficit and photosynthetically active radiation may be suboptimal for these grasses) are stressing the plant and increasing the canopy resistance [e.g., Larcher, 1995;Montaldo et al, 2005].…”

“…With this as motivation, we explore the issue of estimating ET and its relationship with soil moisture, q, for the typical PFTs of an heterogeneous water-limited Mediterranean ecosystem. This is an important ecohydrological issue [Rodriguez-Iturbe, 2000;Albertson and Kiely, 2001;Williams and Albertson, 2004;Montaldo et al, 2005] for both prognostic models, in which predictions of q and land surface fluxes (e.g., ET) are required for a projected radiative and precipitation forcing time series, and diagnostic models, in which land surface fluxes are estimated for a set of observed atmospheric and surface states (q and surface temperature, T s ) using satellite remote sensing observations Kustas et al, 2002;Caparrini et al, 2004;Reichle et al, 2004]. Regarding the diagnostic perspective, the mapping of surface q at high spatial resolutions may be derived from active microwave sensor (radar) observations (up to 10 m of spatial resolution); however the uncertainties on the effectiveness of the radar signal remain large, especially in heterogeneous terrain [e.g., Altese et al, 1996;Mancini et al, 1999;Holah et al, 2005].…”

Comparing two approaches for parsimonious vegetation modelling in semiarid regions using satellite data

“…The proportionality between dry matter production and light absorption is known as light use efficiency (LUE), and this relationship has been widely used in vegetation modelling (e.g. Knorr and Heimann, 1995;Ruimy et al, 1999;Running et al, 2000;Running et al, 2004;Montaldo et al, 2005). Stress conditions, such as water or nutrient deficit, tend to diminish LUE value (Green et al, 1985;Li et al, 2008) so that a correction factor has to be applied in these situations.…”

“…For this reason, a great deal of effort has been made by ecohydrologists in modelling vegetation dynamics along with the hydrological cycle (Zalewski et al, 1997;Hannah et al, 2004). In order to be suitably coupled with operational hydrological models, vegetation models need to only require information commonly available in practical hydrological applications (Arora, 2002;Montaldo et al, 2005).…”

Vegetation dynamics and soil water balance in a water-limited Mediterranean ecosystem on Sardinia, Italy

“…the review of Arora, 2002 andMontaldo et al, 2005). Current VDMs, and representations of ecophysiological processes (photosynthesis, respiration, allocation, phenology) are well reviewed in Arora (2002).…”