Climate controlled root zone parameters show potential to improve water flux simulations by land surface models
Authors: van Oorschot Fransje , van der Ent Ruud , Alessandri Andrea , Hrachowitz Markus 
Published: March 2021

&lt;p&gt;The root zone storage capacity (S&lt;sub&gt;r&lt;/sub&gt; ) is the maximum volume of water in the subsurface that can potentially be accessed by vegetation for transpiration. It influences the seasonality of transpiration as well as fast and slow runoff processes. Many studies have shown that S&lt;sub&gt;r&lt;/sub&gt; is heterogeneous as controlled by local climate conditions, which affect vegetation strategies in sizing their root system able to support plant growth and to prevent water shortages. Root zone parameterization in most land surface models does not account for this climate control on root development, being based on look-up tables that prescribe worldwide the same root zone parameters for each vegetation class. These look-up tables are obtained from measurements of rooting structure that are scarce and hardly representative of the ecosystem scale. The objective of this research was to quantify and evaluate the effects of a climate-controlled representation of S&lt;sub&gt;r&lt;/sub&gt; on the&amp;#160; water fluxes modeled by the HTESSEL land surface model. Climate controlled S&lt;sub&gt;r&lt;/sub&gt; was here estimated with the &quot;memory method&quot; (hereinafter MM) in which S&lt;sub&gt;r&lt;/sub&gt; is derived from the vegetation's memory of past root zone water storage deficits. S&lt;sub&gt;r,MM&lt;/sub&gt; was estimated for 15 river catchments over Australia across three contrasting climate regions: tropical, temperate and Mediterranean. Suitable representations of S&lt;sub&gt;r,MM&lt;/sub&gt; were then implemented in HTESSEL (hereinafter MD) by accordingly modifying the soil depths to obtain a model S&lt;sub&gt;r,MD &lt;/sub&gt;that matches S&lt;sub&gt;r,MM&lt;/sub&gt; in the 15 catchments. In the control version of HTESSEL (hereinafter CTR), S&lt;sub&gt;r,CTR&lt;/sub&gt; was larger than S&lt;sub&gt;r,MM&lt;/sub&gt; in 14 out of 15 catchments. Furthermore, the variability among the individual catchments of S&lt;sub&gt;r,MM&lt;/sub&gt; (117&amp;#8212;722 mm) was considerably larger than of S&lt;sub&gt;r,CTR&lt;/sub&gt; (491&amp;#8212;725 mm). The HTESSEL MD version resulted in a significant and consistent improvement version of the modeled monthly seasonal climatology (1975--2010) and inter-annual anomalies of river discharge compared with observations. However, the effects on biases in long-term annual mean fluxes were small and mixed. The modeled monthly seasonal climatology of the catchment discharge improved in MD compared to CTR: the correlation with observations increased significantly from 0.84 to 0.90 in tropical catchments, from 0.74 to 0.86 in temperate catchments and from 0.86 to 0.96 in Mediterranean catchments. Correspondingly, the correlations of the inter-annual discharge anomalies improved significantly in MD from 0.74 to 0.78 in tropical catchments, from 0.80 to 0.85 in temperate catchments and from 0.71 to 0.79 in Mediterranean catchments. Based on these results, we believe that a global application of climate controlled root zone parameters has the potential to improve the timing of modeled water fluxes by land surface models, but a significant reduction of biases is not expected.&amp;#160;&lt;/p&gt;