Infiltration and surface runoff dynamics on dryland hillslopes: a new method

Drylands cover approximately 41% of the Earth’s land surface (Middleton and Thomas, 1997); a habitat for over 38% of the planet's population (Huang et al., 2017). Understanding the interaction between ground surface characteristics, infiltration and overland flow in this environment is paramount to identifying areas vulnerable to erosion and flash flooding. Currently, infiltration is measured in drylands using techniques which are often not suited to the environment. Existing measurement methods typically cannot be used on steep slopes, and slopes with stone or vegetation cover, without disturbing the natural soil. As well as this, the impact of overland flow is often neglected from measurements. Here, a new method for quantifying infiltration and overland flow is presented: ‘the infiltrator’. The device outputs a pulse of water to the surface, allowing the measurement of runoff dimensions. Soil surface and slope characteristics are also measured with the use of field and GIS based techniques. The methods enable two main research questions to be assessed: (i) the impact of surface cover on surface runoff, and (ii) the influence of surface characteristics on flow concentration. The infiltrator was used successfully on rangeland slopes in a semi-arid environment (Salema, Western Algarve, Portugal), allowing for assessment of infiltration and overland flow, without disturbing the natural soil. Using regression modelling, the results from experimentation using the infiltrator indicted that: (i) infiltration and the nature of surface runoff are strongly related to stone and vegetation cover, and (ii) flow concentration controls include those identified in (i), as well as surface roughness and slope angle. The new method effectively enables the quantification of infiltration and overland flow, whilst remaining representative of the surface. It can be used on slopes up to 40°, and is an inexpensive, quick solution to characterising the vulnerability of dryland slopes to surface runoff and erosion.