</div>
<ol class="arabic simple" start="2">
<li><p>Surface flow equation:</p></li>
</ol>
<div class="math notranslate nohighlight">
\[\frac{\partial Q}{\partial t} + c_{k}\frac{\partial Q}{\partial s} = D_{h}\frac{\partial^{2} Q}{\partial s^{2}} + c_{k}q_{s}\]</div>
<p>The 3-D Richards equation is solved numerically by Galerkin finite elements in space using tetrahedral elements and linear basis functions, and by a weighted finite difference scheme for integration in time. The nonlinear characteristic relat