Dry spell mitigation to upgrade semi-arid rainfed agriculture : Water harvesting and soil nutrient management for smallholder maize cultivation in Machakos, Kenya

Improvements in on-farm water and soil fertility management through water harvesting may prove key to up-grade smallholder farming systems in dry sub-humid and semi-arid sub-Sahara Africa (SSA). The currently experienced yield levels are usually less than 1 t ha-1, i.e., 3-5 times lower than potential levels obtained by commercial farmers and researchers for similar agro-hydrological conditions. The low yield levels are ascribed to the poor crop water availability due to variable rainfall, losses in on-farm water balance and inherently low soil nutrient levels. To meet an increased food demand with less use of water and land in the region, requires farming systems that provide more yields per water unit and/or land area in the future. This thesis presents the results of a project on water harvesting system aiming to upgrade currently practised water management for maize (Zea mays, L.) in semi-arid SSA. The objectives were to a) quantify dry spell occurrence and potential impact in currently practised small-holder grain production systems, b) test agro-hydrological viability and compare maize yields in an on-farm experiment using combinations supplemental irrigation (SI) and fertilizers for maize, and c) estimate long-term changes in water balance and grain yields of a system with SI compared to farmers currently practised in-situ water harvesting. Water balance changes and crop growth were simulated in a 20-year perspective with models MAIZE1&2. Dry spell analyses showed that potentially yield-limiting dry spells occur at least 75% of seasons for 2 locations in semi-arid East Africa during a 20-year period. Dry spell occurrence was more frequent for crop cultivated on soil with low water-holding capacity than on high water-holding capacity. The analysis indicated large on-farm water losses as deep percolation and run-off during seasons despite seasonal crop water deficits. An on-farm experiment was set up during 1998-2001 in Machakos district, semi-arid Kenya. Surface run-off was collected and stored in a 300m3 earth dam. Gravity-fed supplemental irrigation was carried out to a maize field downstream of the dam. Combinations of no irrigation (NI), SI and 3 levels of N fertilizers (0, 30, 80 kg N ha-1) were applied. Over 5 seasons with rainfall ranging from 200 to 550 mm, the crop with SI and low nitrogen fertilizer gave 40% higher yields (**) than the farmers’ conventional in-situ water harvesting system. Adding only SI or only low nitrogen did not result in significantly different yields. Accounting for actual ability of a storage system and SI to mitigate dry spells, it was estimated that a farmer would make economic returns (after deduction of household consumption) between year 2-7 after investment in dam construction depending on dam sealant and labour cost used. Simulating maize growth and site water balance in a system of maize with SI increased annual grain yield with 35 % as a result of timely applications of SI. Field water balance changes in actual evapotranspiration (ETa) and deep percolation were insignificant with SI, although the absolute amount of ETa increased with 30 mm y-1 for crop with SI compared to NI. The dam water balance showed 30% productive outtake as SI of harvested water. Large losses due to seepage and spill-flow occurred from the dam. Water productivity (WP, of ETa) for maize with SI was on average 1 796 m3 per ton grain, and for maize without SI 2 254 m3 per ton grain, i.e, a decerase of WP with 25%. The water harvesting system for supplemental irrigation of maize was shown to be both biophysically and economically viable. However, adoption by farmers will depend on other factors, including investment capacity, know-how and legislative possibilities. Viability of increased water harvesting implementation in a catchment scale needs to be assessed so that other down-stream uses of water remains uncompromised.

On the lifecycle of aerosol particles : Sources and dispersion over Scandinavia

Aerosol particles are likely important contributors to our future climate. Further, during recent years, effects on human health arising from emissions of particulate material have gained increasing attention. In order to quantify the effect of aerosols on both climate and human health we need to better quantify the interplay between sources and sinks of aerosol particle number and mass on large spatial scales. So far long-term, regional observations of aerosol properties have been scarce, but argued necessary in order to bring the knowledge of regional and global distribution of aerosols further. In this context, regional studies of aerosol properties and aerosol dynamics are truly important areas of investigation. This thesis is devoted to investigations of aerosol number size distribution observations performed through the course of one year encompassing observational data from five stations covering an area from southern parts of Sweden up to northern parts of Finland. This thesis tries to give a description of aerosol size distribution dynamics from both a quantitative and qualitative point of view. The thesis focuses on properties and changes in aerosol size distribution as a function of location, season, source area, transport pathways and links to various meteorological conditions. The investigations performed in this thesis show that although the basic behaviour of the aerosol number size distribution in terms of seasonal and diurnal characteristics is similar at all stations in the measurement network, the aerosol over the Nordic countries is characterised by a typically sharp gradient in aerosol number and mass. This gradient is argued to derive from geographical locations of the stations in relation to the dominant sources and transport pathways. It is clear that the source area significantly determine the aerosol size distribution properties, but it is obvious that transport condition in terms of frequency of precipitation and cloudiness in some cases even more strongly control the evolution of the number size distribution. Aerosol dynamic processes under clear sky transport are however likewise argued to be highly important. Southerly transport of marine air and northerly transport of air from continental sources is studied in detail under clear sky conditions by performing a pseudo-Lagrangian box model evaluation of the two type cases. Results from both modelling and observations suggest that nucleation events contribute to integral number increase during southerly transport of comparably clean marine air, while number depletion dominates the evolution of the size distribution during northerly transport. This difference is largely explained by different concentration of pre-existing aerosol surface associated with the two type cases. Mass is found to be accumulated in many of the individual transport cases studied. This mass increase was argued to be controlled by emission of organic compounds from the boreal forest. This puts the boreal forest in a central position for estimates of aerosol forcing on a regional scale.