Plants in the water cycle

By Waiology 30/06/2011

By Daniel Collins

When you think about hydrology, the first things that come to mind are often rain and rivers, not plants. But plants have a significant effect on how much rain actually reaches the rivers.

We see this most readily in the reduction in river flow when grassland is replaced by shrubland or forest. Low flows, mean flow and even low- to medium-size floods are diminished by forests. In an article in 2005, Kathleen Farley and colleagues pooled together results from 26 different hydrological studies. They found that annual runoff was reduced by about 44% when grasslands were planted with forests, and 31% when shrublands were planted. The effects on low flows were similar, though the proportional reductions were greater. We see the similar responses in NZ too. A 2008 MFE report includes a similar review for NZ’s studies.

These effects come about for a number of reasons, but there are several that stand out. The first two take place as the rain hits the canopy. A larger, fuller canopy tends to catch more rain and prevent it from falling to the ground. It instead evaporates back into the atmosphere. This evaporation is sped up by a taller and aerodynamically rougher forest canopy. Another reason is because trees, with more photosynthetically active foliage and deeper roots, tend to transpire more water than smaller plants, sucking more water out of the ground. Lastly, greater vegetation cover can slow runoff and speed up infiltration, which funnels more water either to the aquifer below or to evaporation and transpiration above.

What we don’t entirely know, however, are the details. How much water is intercepted by the canopy of a particular plant species? How much does that plant transpire? And how much does that plant control runoff and infiltration? While we know these numbers for some plants, there are others that we’re a lot less certain about. These details can be important if we need to assess how different land uses affect water availability and aquatic ecosystems.

Filling in these knowledge gaps is the focus of my research within the Waterscape programme. I chose to look mostly at native NZ scrub, like matagouri, manuka and kanuka. We don’t know so much about their hydrological effects, yet they can be a key feature of many of our landscapes. I’ll also look into how carbon farming or ecological succession in the conservation estate affect stream flow, and land cover effects on low flows. But more on that later.ZTMDHSWKS77D