was originally calculated as: (rootlat + rootdrain) *
(1-pcr.exp(-1/rootTT)). This resulted in a continuous outflow of lateral
flow with small values for rootTT, even if there was no lateral flow
generated in the current timestep. This equation has been fixed to:
rootlat * (1-pcr.exp(-1/rootTT)) + rootdrain * pcr.exp(-1/rootTT)