Of the several crop models, the FAO CROPWAT and SMACS models can easily be adopted with very little field data demand. The latter has also been tested for the same study area for observed climatology of 42 years covering the period 1961-2002, under no-climate change conditions. FAO’s CROPWAT is a monthly crop model which considers daily rainfalls applied on selected days of each decade. SMACS is a daily-moisture accounting model adaptable for different crops, and externally coupled with daily rainfall and temperature generation model. Another advantage of the SMACS model is that it easily enables/allows external coupling with GCM outputs with possible disaggregation to daily values using weather generators.Beyond impact assessment, the SMACS model can be used as a decision support system as it can also be usedto calculate water balances such as actual crop ET, excess surface runoff and actual soil moisture besides yield and crop stress indicators. It is therefore suitable to evaluate and investigate the potential of and promote rainwater harvesting and conservation agriculture practices as potential adaptation measures.
The water requirement of the crop at a given time of the growing season is calculated by multiplying the reference evapotranspiration with a crop coefficient, whose values are published by FAO . SMACS model is considered due to easy adaptability to simulate soil moisture balances, crop water demand and stress for current climatology as well as post climate change conditions.In the SMACS model, all precipitation in excess of surface runoff to potential transpiration is used as the environmental indicator of water stress and yield potential -. In the crop models considered in the study, water stress is assumed in the crops to affect growth by limiting photosynthesis in direct proportion to the ratio of actual to potential transpiration. The common approach for estimating crop yield reduction is based on FAO experience, which addressed the relationship between crop yield and water use by proposing a simple equation where relative yield reduction is linearly related to the corresponding relative reduction in evapotranspiration .Future climate changes, as well as differences in climates from one location to another, may involve changes in climatic variability as well as changes in the means. In this study, a synthetic weather generator is used to systematically change the within-year variability of temperature and precipitation, without altering long-term mean values.
For precipitation, both the magnitude and the qualitative nature of the variability can get manipulated. The synthetic daily weather series serve as input to the three crop simulation models. Due to its transboundary linkage and since it exhibits similar agro-ecological characteristics extending and covering Southern Zambia, eastern Namibia, western Zimbabwe and north-eastern Botswana, the site was considered as an ideal site for assessing the climate change impacts of rain fed farming systems. Furthermore, the study site is a vast agricultural area where rain fed agriculture is currently practiced.The area is characterized by the presence of heavy clay soils. It is understood that during the rainy season they are liable to become sticky, waterlogged and poorly aerated, and they become hard during dry periods. They are reasonably fertile and are capable of retaining both water and nutrients . Successful cultivation and good management in some areas have shown that these soils are able to make a significant contribution to food production.Because of low drainage, harvesting rainwater excesses can be practiced in such soil conditions.The climate of the project area, like most of the north and south-eastern Botswana, is sub-tropical and semi-arid. The climate records at Kasane and Pandamatenga compiled by the Department of Metrological Services have been available for the study. The monthly air temperature variation in the project area computed from 1971-2000 recorded data indicates a mean daily minimum and maximum temperature of 15˚C to29˚C, respectively.
The highest temperatures are prevalent during November to March of the year while June and July experience the lowest temperatures.Figure 2 shows the annual temperature variations and general trend in the daily maximum and minimum temperature data of Francistown, a nearby climatic station for the period 1971 to 2000. Wind direction is normally easterly, north-easterly and south-easterly. The incidence of higher winds is greater around September,October and November with average speeds of above 180km/day.Rainfall is characterized by short periods of heavy rain, which cause flush flooding. The men annual rainfall in the study area is generally about 550 mm. The study attempted to establish and understand the degree of susceptibility of crop to failures in terms of soil moisture availability to meet the evapo-transpirative demands of crops studied. Using the SMACS model, the following indices, namely risk, resilience and reliability were used to investigate the available moisture failure rates in rain fed conditions, where rainfed agriculture is practiced in the study site. In this study, daily simulations from climatic data of 1971-2000 were made for the study area, considering commonly grown crops: maize,sunflower,and sorghum.