Hydrology

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Browsing Hydrology by Author "Ocholla, Peter Omondi"

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    The Impact of Flooding Characteristics on Cotton Cultivation in Lower Kano Plain in Nyando District, Western Kenya
    (2010) Ocholla, Peter Omondi; Kelbe, B.; Rawlins, B.
    Flooding continues to be a common environmental hazard in both developed and developing countries. Kenya has not been spared by the destruction that is usually associated with floods. Crops, settlement and infrastructure are usually impaired wherever flooding occur. The severity of damage as a result of floods has been documented to have had a relationship with the flood magnitude, flood frequency and occupation of the flood prone floodplains of large rivers. In the Lower Kano Plains of Western Kenya, damage to crops by floods is exacerbated by occupation of the lower reaches of the Nyando River. This study sets out to assess the impact of flooding characteristics of the Nyando River on cotton cultivation in the Lower Kano Plains. In particular, the study examined the characteristics of the Nyando River Basin with the aim of describing how the river morphometry could have influenced flooding in the Lower Kano Plains. Also investigated, is the change in the flood magnitude and frequency with time and space, and finally, what anecdotal data (perception of cotton farmers) are available to support the assessment of flooding on cotton cultivation. The study deployed both quantitative and qualitative research methods in examining the variability of rainfall and flow and the consequent impacts of flooding on cotton cultivation. Households living downstream in Lower Kano Plains were the target social unit of analysis. Multi-stage sampling technique was used to sample the respondents whom were interviewed through the use of a self administered questionnaire schedule. Descriptive and inferential statistical techniques were used in data analysis. Relevant probability models were used to analyze the flood magnitude and flood frequency. Generally, the findings related to the research question have shown that flooding in the Lower Kano Plains has inhibited cotton cultivation and lower crop acreage. Furthermore, output has significantly declined for the period when spate has either denied farmers the ability for early planting or destroyed cotton already in the fields. The inter-seasonal and intra-seasonal variability of rainfall and flow show their first peaks are dominant in April and May, while October and November present the second cycle. There is however, a shift in the cycles towards August and September, making the annual flow cycle highly variable in terms of decisions for crop growing. These peaks interfere with the cotton growing calendar, and results in delays in planting or destruction of cotton already planted. The results from the spectral analysis revealed a strong annual and biannual cycle of both rainfall and flow, and an oscillating 4 months cycle that exhibited climate instability. A strong seasonal signal was evidenced between 6 and 12 month period that correspond with the flooding peaks. Similarly, wavelet results demonstrated a strong 12 month spectrum of both rainfall and floods with a large frequency of oscillation in the later period of the 31-year time series. The high power band of 2 to 5 years for both the raw and filtered rainfall and flow time series revealed a Quasi Biennial Oscillation (QBO) and a shift in the rainfall and flow cycle. The findings of the correlation also revealed that (r2 = 0.278) rainfall in catchment explains 27.8% of high flow. The rest (72.2%) is attributed to other factors such as anthropogenic or hydro geologic characteristics of the study area. The study area was revealed to be prone to between 3 and 7 years flood return frequency with an average magnitude of 400 m3/sec. Further results showed that out of the 31 years of continuous time series flow the Nyando River recorded 18 years of bankful flow (200 to 387.6 m3/sec). The high frequency of bankful flow illustrates that Nyando River has limited channel capacity and is therefore vulnerable to flooding downstream. The 1 year return frequency characterizes the Lower Kano Plains to crop damage by annual spates, and thus, demands a shift in the cropping pattern and or change of crop variety to those ones that withstand poor drainage. Other factors that have exacerbated the decline in cotton production include poor price of lint, competition from synthetic fibres and rising cost of cotton production. Because of poor remuneration from cotton production, farmers are shifting to the growing of other crops such as rice and sugarcane that are less affected by flooding conditions. The two latter crops are said to be highly viable, cost effective and reliable. The problem of cotton cultivation in the study area is therefore due to variability of hydrologic conditions and economic factors. There is a need for a sound flood mitigation policy as well as the adoption of appropriate agronomic practices that would enhance cotton cultivation and improve output in the flood prone areas for it to be profitable in the present economic climate.