A Big Root Approximation of Site‐Scale Vegetation Water Uptake

Martin Bouda

doi:10.1029/2019ms001806

Abstract

AbstractLand surface model (LSM) predictions of soil moisture and transpiration under water‐limited conditions suffer from biases due to a lack of mechanistic process description of vegetation water uptake. Here, I derive a “big root” approach from the porous pipe equation for root water uptake and compare its predictions of soil moistures during the 2010 summer drought at the Wind River Crane site to two previously used Ohm's law analog plant hydraulic models. Due to a fuller representation of pressure gradients and flows within a complex root system architecture, the new formulation achieves somewhat improved fit and significantly lower bias compared to the Ohm's law analog models. A key advantage of the improved physical representation is the increased robustness of fits and predictions, making it less liable to overfitting. This new mechanistic model advances our understanding of vegetation water limitation at site scale with potential to improve LSM predictions of soil moisture, temperature and surface heat, water, and carbon fluxes.

Highlights

Vegetation access to soil water can play a key role in limiting terrestrial fluxes of water [1], heat [2] and carbon dioxide [3, 4], yet it remains poorly understood at relevant scales
As awareness of the importance of plant hydraulics for ecosystem ecology and earth system science has grown [24], it has become apparent that more mechanistic descriptions of plant hydraulics are needed in land surface models (LSM) to capture vegetation behaviour during drought [25], especially since the frequency of such water-limiting events is likely to increase in the future [26]
A conceptual alternative is provided by the ‘porous pipe’ model of root water uptake [39], which describes the continuous variation of water potential (ψ) along the length (s) of roots taking up water with the differential equation d2ψx ds2

Summary

Introduction

Vegetation access to soil water can play a key role in limiting terrestrial fluxes of water [1], heat [2] and carbon dioxide [3, 4], yet it remains poorly understood at relevant scales. A lack of accurate descriptions of plant water limitation gives rise to significant prediction errors in terrestrial or land surface models (LSM), which aim to predict these surface fluxes as a bottom boundary condition on atmospheric circulation in earth system simulations [5, 6, 7]. A common approach in large-scale land surface, dynamic vegetation or ecosystem dynamics models is to calculate water uptake from each soil layer in proportion to an assumed root length density fraction reduced by a stress factor, which is often a linear function of local soil moisture status [11, 12, 13, 14, 15, 16, 17, 7, 18, 19, 20, 21]. As awareness of the importance of plant hydraulics for ecosystem ecology and earth system science has grown [24], it has become apparent that more mechanistic descriptions of plant hydraulics are needed in LSMs to capture vegetation behaviour during drought [25], especially since the frequency of such water-limiting events is likely to increase in the future [26]

Methods

Results

Conclusion