When it comes to conversion of grassland to shrubs or trees, the typical story goes like this. More rainfall is caught be the foliage and evaporated straight back into the air. Higher rates of transpiration deplete soil moisture faster, and deeper roots inhibit drainage of water from soil to aquifer. This story is typical because it is observed time and time again , but it is not the whole story. Nor is it always true.
A recent paper in Geophysical Research Letters, by Bradford Wilcox and Yun Huang, tells a different story . They document annual streamflow for several rivers in the Edwards Plateau, central Texas. While the region has experienced an expansion of shrubs and trees – AKA, “woody encroachment” – spring-fed river flow has paradoxically increased.
The reason, they propose, is that while woody plants have expanded, grazing has reduced. Livestock trample and compact the soil, and eat the herbaceous plants, both of which reduce infiltration of water into the soil, and ultimately reducing recharge of the aquifers that feed the springs. But as grazing declined, spring-flow seems to have increased. It thus seems that grazing in the Edwards Plateay has a greater impact of river flow than woody encroachment.
While Wilcox and Huang have started to tell a different story, the story is far from over. They have eliminated historical rainfall as the source of the change, but they have not eliminated historical evaporative demand. And while spring-fed river flow has increased, it seems that flow from other sources has also paradoxically increased. If spring-flow increased due to herbaceous plants favouring infiltration, you’d think surface runoff would drop accordingly, but apparently not.
Whatever the answer, the researchers are on the case, and it is fair to say that another exception to the rule of land use hydrology will be found.
 Farley, K., Jobbagy, E., & Jackson, R. (2005). Effects of afforestation on water yield: a global synthesis with implications for policy Global Change Biology, 11 (10), 1565-1576 DOI: 10.1111/j.1365-2486.2005.01011.x
 Wilcox, B., & Huang, Y. (2010). Woody plant encroachment paradox: Rivers rebound as degraded grasslands convert to woodlands Geophysical Research Letters, 37 (7) DOI: 10.1029/2009GL041929