This study investigates rainfall characteristics in the Upper Zambezi River Basin and implications for rain-fed agriculture. Seventeen indices describing the character of each rainy season were calculated using a bias-corrected version of TRMM-B42 v6 rainfall estimate for 1998-2010. These were correlated with maize yields obtained by applying a SVATmodel. Finally, a self-organizing map (SOM) was trained to examine multivariate relationships. The results reveal a significant spatio-temporal variability of rainfall indices and yields, with a gradient from north to south. Yields greater than 1 t/ha are found to be only achievable with rainy seasons longer than 160 days. For shorter durations, the interplay of total rainfall, dry spell frequency and maximum dry/wet spell durations determines agricultural success. Using total rainfall alone or wet day frequency as estimators for yields is insufficient. Alternating wet and dry spells affect yields most negatively. The results have significance in the context of agricultural planning under changing climatic conditions and agricultural planning, as well as for the development of forecasting mechanisms.

We investigate the influence of near-surface wind conditions on subsurface gas transport and on soil-atmosphere gas exchange for gases of different density. Results of a sand tank experiment are supported by a numerical investigation with a fully coupled porous medium-free flow model, which accounts for wind turbulence. The experiment consists of a two-dimensional bench-scale soil tank containing homogeneous sand and an overlying wind tunnel. A point source was installed at the bottom of the tank. Gas concentrations were measured at multiple horizontal and vertical locations. Tested conditions include four wind velocities (0.2/1.0/2.0/2.7 m/s), three different gases (helium: light, nitrogen: neutral, and carbon dioxide: heavy), and two transport cases (1: steady-state gas supply from the point source; 2: transport under decreasing concentration gradient, subsequent to termination of gas supply). The model was used to assess flow patterns and gas fluxes across the soil surface. Results demonstrate that flow and transport in the vicinity of the surface are strongly coupled to the overlying wind field. An increase in wind velocity accelerates soil-atmosphere gas exchange. This is due to the effect of the wind profile on soil surface concentrations and due to wind-induced advection, which causes subsurface horizontal transport. The presence of gases with pronounced density difference to air adds additional complexity to the transport through the wind-affected soil layers. Wind impact differs between tested gases. Observed transport is multidimensional and shows that heavy as well as light gases cannot be treated as inert tracers, which applies to many gases in environmental studies.

<p>Transport of gas components in the unsaturated zone and across the soil surface plays a role for transport of volatile contaminants, gases from pipe leaks or greenhouse gases. When estimating flow rates from the soil into the atmosphere, a good understanding of the transport processes is important. In general, component transport in the gas phase is considered to be mainly due to diffusion. However, the wind field above the soil surface can induce flow into the subsurface and influence transport and mass fluxes.</p><p>We present a study on gas component transport through dry and partially saturated soil into a free air flow above the soil surface, considering gas components of different density. Laboratory experiments in a quasi-2d sand tank were carried out. The tank was placed underneath a wind tunnel, and different wind velocities were used. Gases with different densities were injected with constant rate at an inlet port. Concentration distributions were measured continuously with sensors that were installed inside of the tank. After establishing a steady state concentration distribution, the gas injection was stopped and the decrease of gas concentrations inside the tank was monitored.</p><p>The experiments show that the concentration profiles under steady state gas injection depend on gas density and the different diffusion coefficients. They depend only slightly on the velocity of the overlaying wind field and the influence is mainly seen very close to the soil surface. The transient gas transport out of the soil, however, did not only depend on the different diffusion coefficients, but was clearly influenced by the wind field. The transient 2d concentration distribution fields illustrate that the wind field induced a flow field inside the tank that depends on the wind velocity and the component density and influences the gas component transport. The influence increases under partly saturated conditions.</p><p>To reproduce the transport correctly, it is necessary to capture the coupling between free flow and porous medium flow and the transport in the coupled flow. To do so, we use a fully coupled flow and transport model implemented into the environment DuMu<sup>x</sup> ((Dune for Multi-(Phase,Component, Scale, ...) flow and transport in porous media). It can be shown that including the coupling concept, the main features of the concentration distributions can be reproduced for both the steady state and the transient case. With the model it is also demonstrated, that although advective fluxes inside the porous medium introduced by the wind field (horizontal and lateral) are relatively small in comparison to the diffusive fluxes, they cause relevant changes in the concentration distribution and thus indirectly influence the mass fluxes inside the porous medium and across the soil-atmosphere interface.</p>

<p>Multi-phase multi-component flow and transport models are a key instrument for analyzing and predicting gas transport inside the vadose zone. The soil moisture distribution within the vadose zone varies in time and space. Thus, accurate gas transport prediction relies on the precise knowledge of the saturation-dependency of the transport parameters such as the effective gas diffusion coefficient D<sub>eff</sub>. Although recent advances from typical small scale experiments (diffusion apparatus with typical soil core size of 100 cm<sup>3</sup>) show that D<sub>eff</sub>-saturation(S)-relationships are not only dependent on general soil characteristics such as air-filled porosity and total porosity, but can also be derived from pore network characteristics, such as pore connectivity and geometry, most model frameworks rely on simple empirical formulations such as Millington & Quirk (1961), which find wide acceptance, but have been found to not be universally applicable.</p><p>The current state of research lacks extensive performance tests for the application of D<sub>eff</sub>-S-relationships beyond the small scale and especially for realistic natural conditions, where soil moisture changes with depth and gas transport processes may be more complex than in standard laboratory setups used for the experimental determination of D<sub>eff</sub>, which leads to unknown errors in the numerical prediction of sub-surface gas transport processes.</p><p>We test different D<sub>eff</sub>-S-functions within a multi-phase-multi-component flow and transport model by reproducing a laboratory gas transport experiment, where a tracer gas is injected into a quasi-2D Darcy-scale sand tank with a soil moisture distribution that covers the full range from wet to dry and comparing simulated and measured gas concentrations at several locations within the tank over time. The systematic evaluation of different functions leads to the conclusion that the saturation-dependency of D<sub>eff</sub> in the tested sand follows power law-scaling at low gas phase saturation and linear scaling above, in line with the physically based concepts of percolation theory and effective medium theory and with recent experimental results (Ghanbarian et al., 2018). Other approaches such as (Buckingham, 1904; Penman, 1940; Millington & Quirk, 1961; Moldrup et al., 2000) lead to large errors between numerical and experimental results. We demonstrate that the use of an inaccurate D<sub>eff</sub>-S-function can lead to a misrepresentation of diffusion coefficients by a factor of up to 10<sup>5</sup>, which underlines the need for a correct representation of the saturation-dependency of D<sub>eff</sub> in numerical modeling of sub-surface gas transport.</p>