STUDY OF TWO CATCHMENTS ON THE HILLSIDE OF MT. KILIMANJARO. Paul Christen Røhr & Ånund Killingtveit

Transcription

1 Study of Two Catchments on the Hillside of Mt. Kilimanjaro STUDY OF TWO CATCHMENTS ON THE HILLSIDE OF MT. KILIMANJARO Paul Christen Røhr & Ånund Killingtveit Abstract: Two catchments exist on the hillside of Mt Kilimanjaro instrumented with three gauging stations for discharge measurements. Two of the gauging stations, Charongo and Ngomberi, are located on the same river while the third, Ghona, is located on a nearby river. The results and analysis from the measurements are presented in this paper. The paper begins with an introduction to the Pangani River Basin, emphasising water management and water consumption in the area are the background to this study. The catchments are described and a technical description of the gauging stations and their modes of operation are provided. The results from the 25 years observation period are described and presented in detail for each of the three gauging stations. The specific runoff is calculated for all three stations and a representative discharge episodes are presented and analysed. Alternative calculations of specific runoff, assuming only parts of the catchment contribute to the runoff, are presented. Based on specific runoff from Charongo and assumed water consumption for the agricultural areas, specific runoff for Ngomberi is calculated. The calculated specific runoff is compared with the observed specific runoff. Finally, the paper makes some proposals on how hydrologic modelling can be used for identifying the different elements in the water balance calculation for the area, particularly the groundwater component and water consumption. Field inspection on the hillside of Mt Kilimanjaro indicates that there is no contribution to river runoff from the area above the forest belt. This is supported by calculations based on the observed discharges from the three stations. Calculations of the off-take of water in the agricultural area based on observed river runoff corresponds well with earlier observations of the off-take in the area. INTRODUCTION The 42,2 square kilometre Pangani River Basin, located on the border between Tanzania and Kenya, extends 45 kilometres from Arusha and the 6, masl Mt Department of Hydraulic and Environmental Engineering; The Norwegian University of Science and Technology; N-7491 TRONDHEIM NORWAY; Phone: / ; Fax: ; paul.christen.rohr@bygg.ntnu.no or aanund.killingtveit@bygg.ntnu.no 25

2 26 CHAPTER NINETEEN Kilimanjaro in the north to the river s outlet in the Indian Ocean near Tanga in south-eastern Tanzania as illustrated in figure More than 5% of the area in the basin, consists of arid or semi-arid lowland plains with an annual rainfall of 5-6 mm/year (The United Republic of Tanzania, 1977b). Mt Kilimanjaro Research catchments KENYA NyM Reservoir < N Lake Vi ctori a Kenya ÊÚ Kigom a Lake Tangan yi ka The Pangani Basin Tanzania ÊÚ Dodoma Dar es Salaam ÊÚ Pangani River Pangani Falls Power Plant Zambia Lake Nyasa Mtwara ÊÚ TANZANIA Fig. 19.1: Tanzania and the Pangani River Basin High precipitation is mainly found on the southern slopes of mountain areas with a rainfall of 1,-2, mm/year (Luhumbika, 1999). The population density is about 2.2 million with up to people per square kilometre on the southern hillside of Mt Kilimanjaro. Irrigation of 55 square kilometres of land secures agricultural areas against erratic rainfall on the lowland plains and extends the growing of crops like paddy, maize, banana, beans, coffee, vegetables and sugar cane (Daluti, 1999). In recent years 95 MW electricity generating station has been installed in the lower half of the basin powered by water from the Nyumba Ya Mungu reservoir located in the middle of the catchment with an active storage of 87-mill m 3. The hill slopes of Mt Meru and Mt Kilimanjaro forming the upper part of the river basin have some of the best-developed and well-known traditional furrow irrigation systems in East Africa (Lein, 1998). The extensive network starts far up 26

3 Study of Two Catchments on the Hillside of Mt. Kilimanjaro 27 in the mountains and transports the water along and across the river valleys to more populated areas, acting as a domestic water supply further down into the lowlands. They are also of vital importance for agricultural irrigation. In Tanzania's Structural Adjustment Programme, agriculture, which accounts for half of the GDP and occupies 9% of the labour force, is given high priority. Tanzanian-Authorities (1999) state that the recent increase in agricultural production is less than desired and so greater attention to the its development can be expected which will add to the pressure on the already scarce water resources in the area. In connection with the Pangani Falls Redevelopment Project in the late 198s and early 199s, Water Management was emphasised. Questions were raised in 199 about the simultaneous capacity of production at the downstream hydropower plant and upstream irrigation channels, 5 years before the commissioning of the power plant (Amland, 1995). Awareness of the balance needed between upstream agriculture and downstream hydropower played a part in the establishment of the Pangani Basin Water Office in 1991 for the management of the insufficient amount of water in the area. Tanzanian-Authorities (1999) express that extreme water shortage has been experienced in the past years. This is also reported by Sarmett (1999) with rivers which used to be perennial now completely drying up in the period between September and November, sometimes also until March. Furthermore, according to Luhumbika (1999), Mt. Kilimanjaro generates 6% of the inflow to the Nyumba ya Mungu Reservoir and about 55% of all the surface water in the basin and so changes of runoff pattern in this physically limited area might have repercussions for the whole river basin. Investigations indicate that water availability in the lowland plain has decreased. Based on field studies, Mamfupe (1999) states local farmers reporting that less water is available for irrigating the crops, in the past, there was enough. This could result in conflicts, which are described by Mujwahuzi (1999), who also discusses the underlying causes, for example, unmet needs for water, incompatible objectives high expectations and misperceptions of water availability in the river basin. In this context the idea for research into Water Management and changes in land use developed and resulted in a multi-disciplinary research programme starting in late 1997 with participants from The University of Dar es Salaam and The Norwegian University of Science and Technology. HYDROLOGIC MODELLING AND NEED FOR FIELD DATA In hydrologic modelling work, knowledge about the area to be modelled is essential. This will of necessity varify with the complexity of the model from the simple rational method to the conceptual and physically-based models which aim to represent the processes that actually take place in the catchment. In principle they can have direct measurable parameters represented in the model, although, Refsgaard (1997), states that it is only the main hydrological processes which are represented and require the necessary simplifications for a mathematical description. 27

4 28 CHAPTER NINETEEN For example, the HBV-model is a conceptual model (Killingtveit & Saelthun, 1995) while SHE and MIKE SHE are examples of the physically-based models (Refsgaard, 1996). According to Refsgaard, conceptual models are well suited for rainfall runoff modelling and extension of runoff series from long-term observations. When land use is reflected in the model concept, its change can be reflected in the hydrological response from the model through simulations. Common to all types of hydrological modelling is the need for ground truth data from the actual catchment or from a nearby comparable catchment to calibrate the hydrologic model and verify the simulated discharge. When using a conceptual model, it also requires knowledge about the processes taking part within the catchment in order to have some control over the internal water flows within the model. A complex model representing a large area might give very good results when considering the whole catchment, which shows a natural hydrologic course throughout the year. Taking a closer look at the model and its various sub-parts it might show significant deviations from the actual conditions in the field. The internal errors in the model may cancel each other out. Investigations in the field might reveal new information, give a common explanation of local hydrological phenomena within the catchment and supply numerical facts acting as proof or disproof of these. So far, not much attention has been paid to the validation of internal states and variables in distributed hydrological models in research (Refsgaard, 1997). To get to grips with the hydrological response of a catchment, an understanding of the actual processes that take place within the catchment and determination of quantitative data must be emphasised. One way of doing this is to consider a smaller limited area that less complex and easier to monitor. The information obtained from the smaller catchment, in conjune with previous knowledge, can be used later in the process of understanding the whole catchment. Against this background, with the upstream conditions of great importance for the downstream environment, the need for further investigation of the hydrology in the upper part of the river basin was emphasised in the multi-disciplinary research programme. THE STUDY AREA The hillside of Kilimanjaro can be divided into five ecological zones, namely the lower slopes, forest, heath and moorland, highland desert and the summit (Martin, 2). The surrounding area on the plains below is classified as tropical savannah. The rainfall pattern in the area is bimodal with two rainy seasons, long rains from March to June and short rains from November to December. Human settlement and agricultural activities can be found on the lower slopes between 9 and 1,8 masl in varying degrees. The area above this level forms the Forest Reserve and the Kilimanjaro National Park, with no settlements or human activity except for facilities related to the climbing routes of Mt Kilimanjaro. Martin (2) states that fires, illegal timber collecting, land pressure and over-usage of natural 28

5 3 Study of Two Catchments on the Hillside of Mt. Kilimanjaro 29 resources are among Kilimanjaro s biggest problems. Yanda and Shishira (1999) describe the same problems. In the transition zone between extensive land exploitation and forest, three minor catchments were selected and equipped with stream gauge measurements, (See figure 19.2). In the past, a varying number of gauging stations were operational in the upper part of the Pangani River Basin. According to Sarmett (1999) 34 stations were active in the 195s. The length of the observations varies from a few years in the 195s and 196s until now. However, the number of stations in operation today is M weka ÊÚÊÚ Charongo 1 Ngomberi Maran gu ÊÚ Ghona N Kilometers m e te rs Ice cap/glacier Bushland/alpine desert Natural fores Cultivation with tree crops Mixed cropping Fig. 19.2: The Three Sub-catchments in Pangani River Basin 29

6 21 CHAPTER NINETEEN very limited although rehabilitation of several stations is under way. Most of the operational stations are located the lowland plains below the hill slopes of Mt. Kilimanjaro and Mt. Meru. Very few stations are located on the mountain slopes. According to a survey conducted by Lefstad & Bjørkenes (1997) and Sivertsgaard & Skau (1996), most previous and present stations are located in the lowland area between 7 and 9 masl. Only one station exists at forest boundary at the 1,4 masl, but it has not been functioning for some time. The lowland area is heavily influenced by human activities particular, intensive agricultural production based on irrigation to a certain extent. The flow of water is rather complex, with a large number of off-takes for irrigation and numerous springs of various sizes. Knowledge about the amount of water going into the system from the uphill areas as surface flow and contribution from groundwater sources is poor. In addition to numerous minor springs, groundwater yield is concentrated twothree spring areas, which contribute about 3-5% of annual inflow (The United Republic of Tanzania, 1977b) to the Nyumba ya Mungu hydropower reservoir downstream. In the dry season the percentage is even higher. The origin of the spring-water is not clear. The hills of Mt Kilimanjaro (The United Republic of Tanzania, 1977a) and the hills of Mt. Meru, including the Arusha area, have both been pointed out as recharge areas for the numerous springs in the lowland, but no hydrologic modelling work has so far given any verification as to the extent of the recharge. The United Republic of Tanzania (1977a) provides estimates for spring recharge in various areas based on geological conditions. Yanda & Mpanda (1999) found differences in runoff patterns between ten springs in the hillside of Mt Kilimanjaro. Two patterns are prevalent: high discharge during the dry season and low during the wet season, alternatively low discharge during the dry season and high discharge during the wet season. These findings indicate that the origin of the recharge area and residence time of the groundwater aquifers differs for the different springs. Yanda & Mpanda (1999) did not measure the actual discharge in more than two of the minor springs. Generally the size of the springs varies from a mere ten litres a second to several cubic meters per second. The importance of the different springs described therefore might be difficult to determine. Understanding the process actually taking place in the catchment and determination of quantitative data for spring recharge can be obtained by use of a hydrologic model and water balance considerations. Assuming the hillside of Mt. Kilimanjaro acts as a recharge area for the groundwater reservoir feeding the springs on the plains below, tracing this should be possible. Detailed measurements and hydrological modelling of, e.g., a subcatchment in the hillside of Mt Kilimanjaro, is one approach to the problem. The different contributions to the water balance can be identified and modelled. A recharge component for the groundwater will result. For calibration and verification of such a model, observations of the discharge in the hillside of Mt. Kilimanjaro are essential. Preferably, catchments without human influence on land use should be selected and measured. For obtaining observation series of some length, an early start of the measurements was important in order not to waste too much time in the start-up process. 21

7 Study of Two Catchments on the Hillside of Mt. Kilimanjaro 211 CHARACTERISTICS OF THE SELECTED CATCHMENTS Three different locations were selected for establishing gauging stations based on map studies, discussions with other researchers in the multi-disciplinary project and the regional hydrologist in Moshi, with a subsequent inspection of potential sites. Due to the multi-disciplinary nature of the project, the emphasis was to Table 19.1: Some Information About the Three Catchments Parameter Unit Charongo Ngomberi Ghona Lat dd Long dd Catchment size km Max elevation masl Mean elevation masl Min elevation (loc. of station) masl Length km Started operating 8 th July th July th Oct Recording intervall hour concentrate the research activities in the same physical area. Several conditions for the sites had to be balanced, e.g. the possibility of getting good measurements, stability of riverbeds, security of equipment and accessibility. The three sites selected for discharge measurements are: Charongo River at Mweka just inside the forest boundary Ngomberi River at Kibosho Road bridge Ghona 1 river at Marangu West near Kyala Kilema Road bridge The location of the different gauging stations and their catchments are shown on the maps in figure 19.1 & In table 19.1, various data for the catchments are shown. Figure 19.3 shows the hypsographic curve for the catchments. The Charongo catchment is located inside the Ngomberi catchment, but forms a separate subcatchment upstream. The Charongo catchment stretches from the Uhuru peak in the north southwards to Kifura at Mweka. The upper part of the catchment is formed by parts of the southern side of the snow-covered summit of Mt Kilimanjaro and a highland desert with poor vegetation. The area above 3, masl forms the Kilimanjaro National Park. Between approximately 1,55 and 3, masl a forest reserve limits the activities in the area. No agriculture, collection of firewood, cattle feed or loggings are allowed in the forest reserve. The lower part of the catchment from 1,6 masl constitutes natural forest up to about 2,75 masl where the transition to dense bush 1 The name of the river varies depending on source. The local gauge reader/guard who lives nearby calls it Ghona. On the map sheet 56/4 from Ministry of lands, Housing and Urban Development, Tanzani, 199, the name Ona is used. 211

8 212 CHAPTER NINETEEN Masl 6 5 Charongo Ngomberi Una % Fig. 19.3: Hypsographic Curve for the Three Selected Catchments, Charongo, Ngomberi and Ghona River land starts and form the vegetation which gradually peters out at about 4, masl at which point alpine desert and eventually glaciers are present. The Ngomberi catchment, in which Charongo is located, starts at about 1,1 masl. Mixed cropping and cultivation of grain, vegetables and fruit until about 1,25 masl dominate the lower part. From 1,25 to about 1,55 masl the land is dominated by the cultivation of fruit trees, but coffee is also common here, as the shade from the trees helps to prevent evaporation. The natural forest start at about 1,55 masl and beyond that the land covers are similar to the Charongo catchment. The Ghona catchment stretches from the peak of Mwenzi southwards to Marangu West. The upper parts consist of the rugged rocks of Mwenzi and its many ridges and ravines. The lower part of the catchment from 1,5 to about 1,85 masl is dominated by cultivation of fruit trees and, again, because of shade from the trees, maize is also grown. From 1,85 to about 2,7 masl the natural forest is dominant, with dense bush, forming the upper part of the vegetated zone from about 2,7 to 4, masl. TECHNICAL DESCRIPTION OF GAUGING STATIONS AND MODE OF OPERATION The principal set-up for each gauging station can be seen in figure Each station consists of a data logger, a battery pack and a pressure-bulb sensor, which measure the hydrostatic water pressure, i.e. the water depth. The data watertight logger is housed in an aluminium cover filled with polyurethane foam and is furnished with a LCD display, two control switches and a set of watertight receptacles for various electronic connections. The data logger, operating on a 9-volt battery, has four channels which means up to four different sensors can be connected to each logger. The pressure-bulb sensor with the compensating unit is connected to one of the 212

9 Study of Two Catchments on the Hillside of Mt. Kilimanjaro 213 Compensating unit Datalogger PC for downloading Pressure cell Fig. 19.4: Principle for Each Gauging Station. Logger, Pressure Sensor with Compensating Unit and Battery Pack. A Portable Computer can be Connected to the Logger for Downloading. channels. It measures the hydrostatic pressure caused by the head of water above the sensor from to 5 meters. The influence from the barometric pressure is allowed for the compensating unit, which provides the atmospheric pressure one side of the transducer. The logger stores 1 bits code of the reading from the sensor which implies a reading resolution of approximately.5 cm. Further technical details about the logger, the sensors and general descriptions about pressure bulb sensors can be found in Aanderaa-Instruments (1998, 1997) and Tilrem (1997a) respectively. The logger, the compensating unit, the battery pack and surplus connecting cable are all located inside a steel box fixed at the site. A steel pipe protects the connection cable for the pressure cell. The data obtained by the logger can be transferred onto a portable computer with a programming cable and a simple, standard Windows programme. The logger can also be programmed through the same cable. With the selected logging interval of one hour, the internal memory of the loggers can store more than one year s worth of records before downloading is required. It is also possible to attach additional memory to the logger. Usually downloads have been performed more frequently for secure operation of the loggers, Typically every second month. In addition to the automatic logger, there is a staff gauge at each gauging station. A local gauge reader reads the staff gauge twice a day. He usually lives close to the gauging station and is familiar with the local area. This arrangement takes 213

10 214 CHAPTER NINETEEN care of the security for the gauging station itself. It also provides control of the logger when comparing automatic and manual observations. RATING CURVES The readings from the data loggers represent the water levels at the gauging stations. A discharge curve was developed for each of the three gauging stations. The discharge is measured at different water levels and a stage-discharge curve was developed. Both the current meter and the dilution method have been utilised depending on the condition at the site. The two methods show corresponding results. For measurements with the dilution method, a bulk injection of tracer was performed upstream and conductivity with the respective discharge measured downstream (Logotronic, 1998). Regular table salt was used as tracer for the measurements. The total number of measurements on the stage-discharge curve can be seen in Table For each measurement, two or more measures of the discharge were performed to secure an accurate and controlled result. According to Tilrem (1997b) an instructive number of discharge measurements to obtain a reliable rating curve can be calculated. This is indicated as theoretical in the table. An overall required minimum number of measurements are also given according to the same source. Table 19.2: Number of Actual and Required Discharge Measurements and Percentage of Stage Measurements Within Range of the Rating Curve Calibration Charongo Ngomberi Ghona Number of discharge measurements Required discharge measurements (Tilrem, 1997) Theroretically Recomended minimum Observation within range of disch. measurements, % With the current distribution of discharge measurements on the stage-discharge curve, it can be seen in Table 19.2 that for two stations, 86-98% of the stage observations are within the range of the observations on the stage-discharge curve. For the third station the value is 59%. This can be explained by the conditions in the catchment. The Ghona catchment is more sensitive to surface erosion during heavy rainfalls, which increase the sediment load in the water and complicate the salt-dilution measurements at high discharge when the current meter measurements involve some risk. Surveying of the sites has also been performed for control of the stage-discharge curve. RESULTS Discharge at the Gauging Stations The daily discharge at the three gauging stations can be seen on the plot in Figures 19.6, 19.7 and They show the discharge from the installations up approximately 214

11 Study of Two Catchments on the Hillside of Mt. Kilimanjaro 215 Water level, m.4 Discharge measurements Exponential function Q = 1155 H % of the observed levels within this interval Discharge, l/s Fig. 19.5: Example of Rating Curve. From Charongo River until February 21. On the graphs, a minor number of manual readings have been supplied to fill in missing values. Charongo River The maximum observed daily discharge at Charongo was 397 l/s and the minimum was 12 l/s. Based on these observations, the station had an increasing discharge from approximately mid-march until mid-july, with frequent fluctuations from day to day. The discharges formed a relatively smooth recession curve from mid-july until mid-march with some small peaks in November-December. In both seasons, the minimum discharges occurred in February-March. The observed runoff was considerably smaller in the second season. This due to low rainfall in the region, only about 1/3 of the amount compared with the previous year. The average specific runoff from the catchment was 3.6 l/s*km 2 during the whole observation period. Ngomberi River The maximum observed daily discharge at Ngomberi was 2.9 m 3 /s and the minimum. The station has much of the same characteristics as Charongo, but the increase in discharge starts somewhat later. The recession curve is also steeper. In practice, the 215

12 216 CHAPTER NINETEEN river is dry from mid-october until April for both seasons. In the second season of measurements, the lack of rain in the area caused hardly any measurable runoff at Discharge, l/s Date Fig. 19.6: Observed Daily Discharge at Charongo the gauging station with only a fraction of the previous year s runoff volume. The specific runoff from the upstream catchment was 3.4 l/s*km 2 during the observation period. Compared with the all-year round discharge at Charongo, the discharge at Ngomberi seems to be a limiting factor for water consumption Discharge, l/s Date Fig. 19.7: Observed Daily Discharge at Ngomberi 216

13 Study of Two Catchments on the Hillside of Mt. Kilimanjaro 217 Ghona River At Ghona the maximum observed discharge was 1. m 3 /s and the minimum.14 m 3 /s. The recession curve has much of the same characteristics as the two other stations. It continues until end of March 1999, when a sharp rise in the discharge 12 1 Discharge, m 3 /s /7/1998 1/3/ /1/1999 3/6/2 1/3/21 Date Fig. 19.8: Observed Daily Discharge at Ghona takes place. Frequent fluctuations occur until mid-june and the relatively smooth recession curve continues until the end of April. In the 2 season, there is only a fraction of the previous season s runoff with no clear peak discharge as in the previous year. The specific runoff from the catchment is 6.9 l/s*km 2. Differences in Discharge - Charongo vs. Ngomberi As can be seen from Fig. 19.9, the two catchments behave somewhat differently behaviour during the first part of the long rains. The measurement of discharge at Charongo and Ngomberi and the precipitation at Charongo during the start of the long rains in the 1999 season shows that more than 15 mm of rain fell during March before any discharge was traceable at all at Ngomberi on 27 th March The one-hour resolution of the raw data makes it possible to observe the floodpeak travelling from the gauging station at Charongo to the gauging station at Ngomberi. One incident from the long rains in spring 1999 is shown here, namely the 2 th - 21 st April 1999 peak in figure The precipitation measurements are taken at the gauging station at Charongo. About ¾ of the Ngomberi catchment is located above the rain gauge and ¼ below. The rain gauge used is a standard 5 / 127mm and the manual readings are performed at 9 am daily. The rainfall is indicated in a separate plot on the hydrograph in figure During a three-day period from 19 th -21 st April a total of about 1 mm of precipitation was observed. Previously, 217

14 218 CHAPTER NINETEEN Discharge Charongo River Discharge Ngomberi River Precipitation at Charongo Fig. 19.9: Discharge at Charongo and Ngomberi and the observed precipitation at the start of the long rains in the 1999 season. during March and April more than 4 mm of precipitation had been observed, so the soil moisture in the catchment was saturated to some extent. The flow at the gauging stations was very low with 34 l/s at Charongo and 44 l/s at Ngomberi. For Discharge at Charongo, l/s 12 Precipitation, mm Discharge at Ngomberi, l/s 3 Charongo 25 Ngomberi : : : : : Fig. 19.1: Episode with Heavy Rains and Sudden Increase in Runoff in the Charongo-Ngomberi Catchment 218

15 Study of Two Catchments on the Hillside of Mt. Kilimanjaro 219 about one hour, the flood increased 2 times at Charongo and 5 times at Ngomberi. The delay in maximum discharge from Charongo to Ngomberi was about 4 hours. Other events with heavy rainfall incidents over short periods show some of the same characteristics of an increase in discharge, though not to that extent. The differences in the observed specific runoff from the Charongo and Ngomberi catchments for the whole observation period can be seen in figure It shows that the observed specific runoff seems to be higher for the Charongo catchment during most of the dry season and for the two observed short-rain seasons. During and after the long rains, from about mid-april until mid-august, the specific runoff is higher at Ngomberi than at Charongo for the first whole season of observations. 6 Daily specific runoff, l/s*km Charongo Ngomberi 1/7/1998 1/3/ /1/1999 3/6/2 1/3/21 Fig : Specific Runoff from the Ngomberi and Charongo Catchment DISCUSSION Local Runoff Pattern During the long rains in the 2 season the precipitation was considerably lower than in the previous year. At Charongo the precipitation was only about 1/3 compared with the previous year. This might explain the almost total absence of observed runoff at Ngomberi and the reduction at Charongo which in turn accounts for the failure to cultivate crops during the dry season, due to lack of rain. The major runoff season for the catchments is from April to August/ September. According to studies of topographical maps, the two catchments range 219

16 22 CHAPTER NINETEEN from 5,895 masl down to the gauging stations. They have great variations in vegetation, from ice cap to evergreen forest. During two field inspections along the river, in July 1999 and in June 2, it was found that, with increasing altitude, the surface runoff gradually decreased along the Charongo River. From approximately 2,8 masl the surface runoff was not significant or absent in the Charongo catchment. The inspections in July 1999 and June 2 took place just after the long rains. When the area should have been saturated and slowly draining away. There was no sign of a normal river/stream bed, but more a trench, which seemed to have a more infrequent discharge pattern. Since no discharge was observed during the inspection just after the long rains, it is probable that there was none during the rest of the year. The inspection indicates that the contribution to direct surface runoff from the upper bush land/alpine desert and melting ice cap is small. Table 19.3: Area of Different Vegetation Zones in the Catchment Charongo Ngomberi Una Absolute areas "Non-contributing" area above forest, km Forest area, km Agricultural area below forest, km Total, km Contributing area, km 2 (Forest + agricultural) Relative areas "Non-contributing" area above forest, % Forest area, % Agricultural area below forest, % 2 11 Total, % In figures and 19.13, an alternative calculation of the specific runoff from the two catchments is shown. Here the forest area is considered to be the only area contributing to the observed runoff from the catchment. Only the area for the forest zone in the two catchments is used when calculating the specific runoff. From figure 19.13, the period from April to August 1999 can be seen in detail. The similarity in relative runoff from the two catchments is noticeable compared with the result for the whole catchment as shown in figure Some data for the different vegetation classes in the catchments can be seen in Table In absolute terms the forest area in the Ngomberi catchment is about 4 times bigger than in the Charongo catchment. The absence of runoff at Ngomberi during most of the dry season can, for example, be explained by off-takes for irrigation and domestic water supply between the two gauging stations. In addition, the area between the two stations, which contributes to surface runoff, can help to explain some of the discrepancies. Based on the observations from the field, an example of 22

17 Study of Two Catchments on the Hillside of Mt. Kilimanjaro Daily specific runoff, l/s*km Charongo Ngomberi 1/7/1998 1/3/ /1/1999 3/6/2 1/3/21 Fig : Specific Runoff from the Ngomberi and Charongo Catchment When the Upper Part of the Catchment is not Contributing to the Runoff 12 Daily specific runoff, l/s*km Charongo Ngomberi 1/4/1999 1/5/1999 1/6/1999 1/7/1999 1/8/ /8/1999 Fig : Specific Runoff from the Ngomberi and Charongo Catchment when the Upper Part of the Catchment is not Contributing to the Runoff. Details are Shown for April-August 1999 the development during the wet season can be as follows: During the dry season, most of the water in the river between Charongo and Ngomberi is used by people along the river for various purposes with irrigation probably being the major reason. During the first part of the wet season, most of the rain is absorbed by the soil. Figures 19.9, 19.6 and 19.7 show that a considerable amount of precipitation occurs before the surface runoff increases. When the soil is saturated, the need for irrigation is less and more of the water is left in the river. As the wet season ends and the soil dries up, the irrigation requirements increase and with them the off-take of water 221

18 222 CHAPTER NINETEEN between the gauging stations. The decreasing discharge and increasing off-take in the recession period will little, by little lead to the total absence of water at the gauging station at Ngomberi and a dry river shed. WATER CONSUMPTION A water balance can be set up for the Ngomberi catchment based on the contributing areas as shown in Table 19.3 and the observed specific runoff from the Charongo catchment. It is assumed that the specific runoff from the forest part of the Ngomberi catchment is similar to that Charongo catchment. The runoff from the agricultural area in the Ngomberi catchment is assumed to be relatively the same as the forest part of the Charongo catchment, but is compensated for by lower precipitation. The water balance will then as follows: Where: Q n = Calculated discharge at Ngomberi Q c = Observed runoff at Charongo Q nf = Runoff from forest part of the Ngomberi catchment Q nag = Runoff from agricultural part of the Ngomberi catchment Q offtake = Off-take of water in the agricultural part of the Ngomberi catchment During the water balance calculation for Ngomberi, an off-take of 26 l/s*km 2 for the agricultural area was found to give the best fit. With the agricultural areas given in Table 19.3, this represents a total off-take of about 275 l/s on the section between Ngomberi and Charongo. A survey conducted by Pangani Basin Water Office in 12 Daily specific runoff, l/s*km Calc.spec.Ngomberi Obs.spec.Ngomberi Fig : Calculated and Observed Specific Runoff for the Ngomberi Assuming the Upper Part of the Catchment is not Contributing to the Runoff 222

19 Study of Two Catchments on the Hillside of Mt. Kilimanjaro indicated an off-take of about 35 l/s. The deviations can explained by the uncertainties in the estimation or changes in the size of the off-takes. The result from the water balance calculation can be seen in figure and It shows the observed and calculated specific runoff from the Ngomberi catchment, and how well fit. With a theoretical demand/off take of 26 l/s*km 2, the calculations show that the demand for off-take is bigger than the available supply for parts of the year. 12 Daily specific runoff, l/s*km Calc.spec.Ngomberi Obs.spec.Ngomberi Fig : Calculated and Observed Specific Runoff for the Ngomberi Assuming the Upper Part of the Catchment is not Contributing to the Runoff. Details for April-August 1999 are Shown in the Figure The demand for water used for the calculations in figure may be difficult to estimate. After a considerable time or drought during the dry season, the demand for water and therefore the off take of water is at its height with gates open at the maximum. When the rain starts and following discharge occurs, the off-take from the river will still be big. However, in the wet season, the need for water decreases, and the off-take is adjusted to a lower level. Once the soil starts to dry up, the demand for water increases again and the off-takes are regulated upwards. The calculated water balance for Ngomberi in figure also has some peaks in the dry period during the last half of 1999 and first half of 2. During the calculations, the runoff from the agricultural area is assumed proportional to the runoff from the forest area. This may not be correct for the dry period and small deviations will be introduced. 223

20 224 CHAPTER NINETEEN CONCLUDING REMARKS The result from the calculation above can be used for estimating the water use in the area and then as a component in a distributed hydrologic model describing the water demand for various areas. This hydrologic model together with a component describing the contribution to the groundwater recharge in the area, can support the theory presented in this article and give a good overview of the water balance in the area. A necessary base for the hydrologic modelling work is the ground truth data obtained from the described gauging stations. Land use and water demand can be represented in the model and changes in land use can be traced in the hydrological response from the catchment. Some ideas about water demand and the relation between land use and water demand based on the observations in the field have been presented in this article. The ideas develop from each other and end up with the demand for hydrological modelling supporting the theory presented in the article. Future such could work can contribute to the theory about water use and groundwater recharge, but a necessary base for this work is the ground truth data obtained from the measurements described in the article. Land use can be represented in the model and changes in land use can be traced in the hydrological response from the catchment. REFERENCES Aanderaa-Instruments. (1997). Water Level Sensors (WLS) 319/319A (Data sheet for equipment). Bergen, Norway: Aanderaa Instruments. Aanderaa-Instruments. (1998). Specific In formation for Data-logger 3634 (Manual for equipment). Bergen, Norway: Aanderaa Instruments. Amland, B. A. E. (1995, 27th July 1995). Conflicts over Pangani dampen enthusiasm for hydropower in Tanzania. Development Today. Nordic Outlook on Development Assistance, Business & the Environment, 5, 1, 6-7,12. Daluti, R. L. (1999). Irrigation Development and Water Management in Pangani River Basin (Workshop contribution ). Bahari Beach, Dar Es Salaam, Tanzania: Idara ya Maji, Moshi. Killingtveit, A., & Saelthun, N. R. (1995). Hydrology. (1 ed.). (Vol. 7). Trondheim: The Norwegian Institute of Technology, Division of Hydraulic Engineering. Lefstad, L., & Bjørkenes, A. (1997). Diploma Thesis in Water Resources Planning. Hydrological Studies in The Upper Pangani River, Tanzania. Unpublished Diploma Thesis, The Norwegian University of Science and Technology, Trondheim, Norway. Lein, H. (1998). Traditional versus modern water management systems in Pangani River Basin, Tanzania. In L. de Haan & P. Blaikie (Eds.), Looking at Maps in the Dark. Directions for Geographical Research in Land Management and Sustainable Development in Rural and Urban environments of the Third World (pp ). Utrecht/Amsterdam: Royal Dutch Geographical Society, Faculty of Environment Sciences, University of Amsterdam. Logotronic. (1998). Qtrace - Discharge measurement by the salt tracer dilution method (Document: doc, Version 1., 7/98). Vienna/Austria: Logotronic - GmbH. Luhumbika, B. A. S. (1999). Water Management in Pangani River Basin (Workshop contribution). Bahari Beach, Dar es Salaam, Tanzania: Idara ya Maji, Moshi. 224

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