Surface Water Quantity Baseflow About

What is baseflow?

Baseflow is the proportion of streamflow that is sustained in between precipitation events. In many, but not all cases, this will be through seepage from groundwater. During the dry season & especially in water scarce regions, baseflow may be the primary or only source of surface water for human consumption, for support of aquatic biota, and assimilation of organic-waste discharge.

What is forest's influence on baseflow?

Simply put: it is complex! Forest-atmosphere interactions leading to precipitation enhancement are currently debated. And, catchment-scale studies show that forest-water interactions cannot be generalized. Deforestation/afforestation usually leads to an increase/decrease in annual streamflow because forests tend to use more water compared to lower vegetation. Overall, forest water use is determined by plant species, seasonality, soil depth and water content. Some studies reveal an increase in streamflow variability following deforestation. This variability implies higher peak flows during heavy rainfall and lower base flows during dry spells, and this variability is based on a change in soil infiltration capacity following loss of forest cover.

What about Cloud Forests?

A class for special consideration are Tropical Montane Cloud Forests (TMCFs). TMCF's increase in fog persistence with an increase in elevation, leads to distinct hydrological and ecological functioning. Fog reduces incoming solar radiation, increases leaf wetness frequency and decreases the atmospheric vapour pressure deficit, all of which reduce transpiration. Simultaneously, fog interception by the forest and foliar water uptake can constitute relevant water sources to the system. Low TMCF transpiration and the water input by fog interception may support the idea that catchment streamflow decreases following TMCF deforestation. However, the few studies comparing streamflow between forested and deforested catchments showed no differences or even lower streamflow for forested catchments. It should be noted that none of these studies addressed uncertainty in estimates of hydrological fluxes. The mismatch between the observations on specific hydrological processes (i.e. transpiration and fog interception) and the catchment streamflow responses to deforestation are likely caused by: 1) low cloud water inputs to the studied catchments; 2) high water interception by rough grasslands and shrubs that is comparable to those of forests; 3) the narrow distribution of high fog intercepting areas in contrast to the broader distribution of lower areas with no or low cloud water input; and 4) geological and topographical differences between forested and deforested areas. These results highlight the difficulties in defining the hydrological functioning of TMCFs and suggest that their role in sustaining water yields depends on site-specific conditions. TMCFs are also particularly vulnerable to upwind land cover changes because fog persistence depends upon the temperature and moisture content of the air masses. Therefore, it is essential to distinguish between coastal and continental TMCF, as coastal TMCFs’ moisture conditions are determined by the ocean influence, whereas continental TMCFs’ moisture conditions will mainly depend on the upwind land cover.

The text above is derived from a paper by Ramírez et al. 2017

How is baseflow measured?

Monitoring of baseflow can be considered in the following ways. (A few of these methods that may be used in our field-projects are added as sub-pages):

1. Baseflow separation using tracer-based methods, conductivity mass-balance, hydrologic simulation methods, software-based approaches or graphical methods.
2. Baseflow statistics using long terms streamflow time series to determine low flow values.
3. Sources & flow paths by using solutes.
4. Water fluxes between surface water and ground water using Flow-Net Analysis, Hydraulic Potentiomanometer or Seepage Meters.