Land & Water Processes
In an academic environment many people believe that when we feed more detailed process
knowledge into models, this will result in an outcome that helps to make farming more sustainable and profitable. In my opinion this is a myth.
Let me explain this. Water erosion (Picture 1) is the most important degradation process of the 10 degradation processes distinguished by the European Union. The impact of rain drops on the soil surface causes splash (Picture 2) of soil particles. This is an important erosion process at the point scale. However, if the splashed particles are not transported there
will be no detectable erosion at higher scale level. As one of PhD’s states: ‘no runoff, no soil loss’. This implies that, in order to quantify erosion, we have to study the field water balance, i.e. the flow of water over or into and through the soil (Picture 3). Of crucial importance thereby is the role of the soil surface with its highly dynamic structure due to soil tillage, trampling by cattle, crusting due to disaggregation (Picture 4), soil water repellency and many more processes which many still remain unknown.
In spite of the many years that soil physicists have spent on these phenomena, there can be only one conclusion: to date we do not understand or cannot adequately describe what happens with energy, liquid and vapour flow in the top mm of the soil. Scale and landscape much affect land degradation processes. I have always spent a lot of research on processes, both in the field and in the laboratory with help of rainfall simulators (picture 5 and Ref. 1), wind tunnels (Picture 6) and remote sensing. Conclusions from the first of these detailed measurements in the lab with double gamma rays in 1974 , do not differ from the last in 2012: many basic questions on soil processes still remain unanswered.
But even when we know more at the mm scale, the conceptual and contextual understanding of how we can upscale this knowledge to the physics at landscape and watershed scale (Picture 7 & 8) is still inadequate. In my opinion there is no need to know more physical details. For an applied science as Land and Water Management, current soil physical knowledge is sufficiently mature. Simple, semi-empirical equations work very well in most circumstances and are still widely used. It is not ever more detailed soil physics but more conceptual understanding and modelling at higher scale levels that will bring us further.
Land degradation might be a physical problem, the cause is the continuous decrease of soil organic matter in cultivated semi-arid soils, Ref. 2
Ref.2 Ali, M. ; Sterk, G. ; Seeger, K.M. ; Stroosnijder, L. (2012). Effect of flow discharge and median grain size on mean flow velocity under overland flow. Journal of Hydrology 452 . - p. 150 - 160.
Ref.1 Stroosnijder, L., 2014. African Hungry Soils. Poster session SSS0.5, Abstract EGU2014 – 15864. European Geophysical Union, General Assembly 2014, 27 April – 2 May 2014, Vienna, Austria
There is no research without the use of models, whether conceptual (usually simple) or deterministic (usually very complex). This implies the need for data. Good data are scarce and data collected in the past are badly stored or available. Hence (new) measurements are needed. This is easier said than done. Measurements are time consuming (in comparison with running a model) and expensive. Much technical staff and support has disappeared in research groups. Hands-on training in methods and equipment falls short in formal education programs. So, video training (Youtube) is gaining importance: watch my video about the determination of the soil’s evaporability: https://www.youtube.com/watch?v=k-PAw94h3Qw.
Most equipment is not available at the commercial market but locally made by researchers. This implies little standardization. In the past the focus was on details and accurate point measurements (mainly to understand processes). Nowadays the focus has shifted from fundamental research to more applied research. There is a need for cheaper instruments like sensors that can be used in larger numbers over a wider area, this is particular true for moisture sensors. Since getting real time information over larger areas is popular, UAV’s and drones are used. With respect to measurements it is crucial to remain critical and be careful with extrapolations Ref. 1 & 2).
Ref. 1 Stroosnijder, L. (2005). Measurement of erosion: Is it possible? Catena 64 (2-3). - p. 162 - 173.
Ref. 2 Fleskens, L. ; Stroosnijder, L. (2007). Is soil erosion in olive groves as bad as often claimed? Geoderma 141 (3-4). - p. 260 - 271.