Masahiko TOMITA, Masaru TOYOTA*, Hajime TAKENAKA, Mitsukata Suzuki**, K

熱帯農業(Japan・J.Trop.Agr.)27(4):244-258,1983

Land Use Strategy in Tropical River Basin from the Viewpoint

of Soil Conservation*

-Watershed Management in Lampung Province , Indonesia-

Masahiko TOMITA**, Masaru TOYOTA***, Hajime TAKENAKA**, Mitsukata SuzuKI****, K. E. S. MANIK***** and B. ROSADI***** **Faculty of Agriculture , The University of Tokyo, Tokyo 113 ***Faculty of Agriculture , Niigata University, Niigata 950-21 ****Institzcte of Agricultural Engineering , University of Tsukuba, Sakuramura Nihari-gun, Ibaraki 305 *****Fuculty of Agriculture , University of Lampung, Province Lampung, Indonesia

Abstract

Significant developments of Lampung agriculture have been caused by the artificial improvements of regiomal ecosystems due to the recent large-scale hydrological construction. But the agriculture at the region is so stable and sometimes suffers drastic damage because these artificial improvements have not yet been accustomed to the nature and human life of the region. So, after some consideration on regional agro-ecosystem and present condition of watershed, a simula tion model for watershed management was developed and adapted to the Seputih river basin. It is resulted that in order to maintain the stability of regional agriculture and hydro-facilities some improvement should be made as written in Section 5. The rivers flowing through Lampung carry many suspended soil particles which eventually enter canals as sediment and close water passages. Rainfall is the first agent to produce soil erosion which makes river water turbid. Therefore, appropriate management of watershed is essential for the conser vation or the improvement of regional agro-ecosystem.

1. Agro-Ecosystem in the Province of Lam season, so that dry farming exceled in traditional pung agriculture. These are the typical characteristics (1) Geographical aspect of Lampung Pro of tropical area. vince (2) Agriculture in Lampung Province of Lampung is located in far south There are 430, 000 ha of dry farms (for rubber, part of Sumatra Island. Micro-landf rom of south- coconut, palm, coffee, clove, pepper, cassava, etc.) ern part of the Sumatra, is divided into 4 types, and 260,000 ha of paddy fields in Lampung i, e., mountain, peneplain, river and coastal plain today. However more 490,000 ha of primeval according to Kaida,1) and the central greater forest are lying for the objects of agricultural part of Province of Lampung belongs to the development besides 300, 000ha of reserved forest peneplain in this classif iccation. A heavy rainfall and 340, 000 ha forestry-use area.2 around 1, 900 to 3, 200 mm per year erodes the ) The agricultural development in Lampung can accumulation of volcanic ash soils and contributes be divided into three stages. Stage I is before to shape micro-landf orm. Large rivers flowing the 18th century, and upland cultivations of through Lampung, Way Seputih, Way Sekam- kapok, banana, cassava, cereals and rice prevailed pung and Way Tulangbawang, originate from under shifting cultivation. Stage II is the 19th the Barisan mountains and flow down eastwards century, and shifting cultivation prevailed in most through the peneplain and coastal plain to the parts of the peneplain, while coffee, pepper and Java sea. Annual rainfall of Lampung is enough rubber plantations were newly developed. Stage for rice planting, but most of it occur in wet III is the 20th century, then goverment promo- * Received 22 December 1982 . ted transmigration to Lampung and constructed TOMITA et al.: Land Use Strategy in Tropical River Basin from the Viewpoint of Soil Conservation 245

Tabe 1 Basin land use and SS of river water tion). The harvesting area of Lampung are shown in Table 1. The area of paddy field (under waterlogged conditions) dominates, but is still small in comparision with the total paddy area of the Indonesia. The other main crops of Lampung are upland rice, cassava, coffee, clove, pepper and soybean. National policy and economy impact the agricultural development of a country. But, one of the direct factors of the agriculture development is environmental conditions for the plant growth. Plant growht is affected by light, fertility of soil, and water. From the viewpoint of hydrology, either excess or shortage of water influences plant growth and agricultural development. The typical conditions and land use of excess and shortage of water in Lampung are * no data as land use map , but it eeems that about 10% of catchment is used for dry farming in ac- as follows ; cording to observaiion, (i) Excess of water results in waterlogged ** sub-basin No . equal to Block No. in Tank Model conditions. The sites under these conditions are, Simulation e. g., humid lowlands of the central Way Jepara, the river mouth of Way Seputih, and the alluvial irrigation systems for settlements, one of which site of the Barito river in Kalimantan. Farm was the huge irrigation canaal of Way Sekam- lands hardly exist in these sites because there pung which was constructed in 1935. exist no drainage canals except the Way Jepara In 1977, the total population of Lampung short-cut. reached 3.7 million (2.8 % of the total popula- (ii) In Tropical region it has considerable tion) and the population in rural villages was rainfall around 2,000 mm in the wet season but 3.1 million (85 % of the total Lampung popula- not so much in the dry season. The shortage

Table 2 Type of irrigation system for lowland rice fields in Lampunp 246 熱帯農業27(4)1983

of water in the dry season restricts the Lampung of irrigation system

agriculture. In the scant rainfall area, they chose Every plant species repeats its growth cycle drought resistant crops such as cassava that is in each generation. The accumulated biomass of generally grown in the peneplain. Other crops a plant species increases as the succession ad- which require more rainfall like tea are grown vances, but the regrowth of the species declines

on the hill and mountain slopes which are gradually and finally ceases completely, which favored with rainfall. In spite of the shortage is called •gclimax plant growth•h from the view-

of water both farmers and national economy point of succession. When the generation reaches require rice production in paddy fields. An the climax condition, it is called climax ecosys- irrigation system is then a prime tool for this tem. And, when the first generation is repeated purpose. And, the planning of an irrigation by artificial harvesting, it is called agro-ecosys- system depends basically on the location of the tem. Naturally, agro-ecosystem produces a new water sources available. Because Lampung lo- biomass increment in each generation which

cates in the peneplain and can not gather enough climax ecosystem does not. In extensive concept, rain water within its catchment area, water the elements involved in the agro-ecosystem are

should be carried through canals from the out- crops cultivation and social conditions such as

side. Therefore, the development of canal net- economy, policy and human organization. The works also results in the agricultural develop- relation between these elements in the agro-

ment. ecosystem of Lampung was studied according to According to the whole Lampung survey of the approach of R. P. Hart.3) And the ecosystem hydrological condition and its support engine- of farm lands as an subsystem in such system

ering of paddy field, irrigation systems for the is shown in detail in Fig. 1. This is an agro- paddy fields in Lampung are classified into the ecosystem in limited or physical concept. As to following seven types ; I, II, IIIa, Tub, IVa, IVb the above discussion, the relation between agro-

and V (Table 2). ecosystem and social conditions outside is shown

The construction cost of each type increases in Fig. 2. The described relation helps to under- from I to V and the stability of water supply stand the meaning of an irrigation system in the

is also improved correspondingly. Therefore the agro-ecosystem in limited concept. The condition

types from I to V give an index of the agricul- of a new irrigation system causes an increase

tural development from the viewpoint of water in farm land area. And, the improvement of an use stability. And the major type of paddy existing system increases the productivity of

fields found in Lampung is IVb because of farm land. This is the reason why the big irri-

recent large irrigation projects. gation system constructions are basic in agricul- Agricultural development depends largely on tural development. But excessive increase in

reasonably managed irrigation systems. But the population may cause forced land clearing and examples obtained in Lampung suggest the fol- consequently affects the homeostasis of environ-

lowing two problems, One in the lack of ex- mental ecosystems. Therefore, the number of perience. Irrigation types of IVa, IVb and with settlers must be limited to a permissive level. artificial canals are still very rare and most of And in the paddy fields with irrigation types the farmers have not experienced the manage- of I to IIIa, a stable agro-ecosystem in a limited

ment of organized and advanced irrigation sys- concept prevails while its productivity is on a

tems, The other one is the difficulty in mobili- low level. On the other hand, in the fields with

zation. The transmigrated populations have irrigation types of VIb and V, advanced types,

originated from various regions. This fact hinders the following problems seem to be unavoidable.

communication and cooperation among farmers One problem is that these fields are derived

which are necessary for the management of from poor soils such as podzols and latosols, so irrigation systems. that sufficient fertilizers may be required for

These problems are probably common among local varieties too. Another one problem is that most developing countries and should be solved. excess of some solutes may arise because canals (3) Relationship between the structure of must carry the solutes deriving from the eroded agro-ecosystem and the development soils and cause water pollution of rivers. TOMITA et al.: Land Use Strategy in Tropical River Basin from the Viewpoint of Soil Conservation 247

Fig. 1 I/o and relation between elements of Agro-ecosystem in farm land (Agro-ecosystem in narrow sence)

2. Consideration on the Watershed from the intensive farming operations on the uplands ; Viewpoint of Soil Erosion and (b) decrease in water-holding capacity of the On the 28 of January 1979 a national railway forests. After shifting cultivation, soil fertility bridge crossing over the river Way Seputih and of the forests reduced and therefore frail plants the dike near the headwork of Way Seputih now dominate this region, resulting in much irrigation system were destroyed by a flood. less waterholding capacity. Many floating materials such as dead trees were (1) Present condition of basin Way Seputih caught by bridge pylons and caused the rise of (a) Condition of soil suspension (ss) in River river water level. This was the cause of the Way Seputih destructions. SS in river water is an important indicator The main causes of the ruin of basins like of soil erosion level in a catchment area. Meas- above are as follows : (a) soil erosion caused by urements of SS in Way Seputih have been 248 熱帯農業27(4)1983

Fig. 2 Relation between Agro-ecosystem and outside condition taken since 1976 when an irrigation canal was opened to use. Solid line in Fig. 9 shows the dayly data of SS at the head of the network. In this figure the maximum SS is about one thousand, but minute observation in flood shows that it easily rises over three thousand. From these data it was found that : a) In flood time the value of SS goes up suddenly, but comes down slowly with discharge, b) Even in the time of low water level, SS is more than 100 ppm. However above data does not represent the whole river but only Sugalamidar point which the head-work is located. Then we observed SS at six points distributed along the main stream of Way Seputih (Sugalamidar, Tatayan, Ajibaru, Gunungsugih and Rumbia) and five principal branch rivers (Sungke, Tatayan, Wayah, Fie. 3 Chance of SS alone W. Senutih Bungur and Billu). Water sampling was done on the 3rd and 4th of September 1981, then lower than that of Ajibaru, which suggests a these samples were analyzed by the staff mem- soil loss from the dense dry field zone along bers of the University of Lampung (UNILA). the Way Seputih because there is no other Fig. 3 shows the distribution of SS of Way considerable inflow river. Seputih. The results are as follows : a) In (b) Sedimentation on river bed comparison with the solid line in Fig. 9, it can Mr. Hamamori who worked in the DPU be said that we grasp the condition of high (Dept. of Public Works) Water Resource Bureau concentration, b) From the upstream (the head) and has been concerned with flood control pro- to the downstream, SS increases, but after paddy ject in Indonesia summarized the characteristics field use, downstream SS becomes very low, of rivers in Indonesia connecting soil loss to proving the clarif icative effect of paddy field, topographical factors. c) SS in the branch river above Ajibaru is (1) There is much soil demand from active TOMITA et al.: Land Use Strategy in Tropical River Basin from the Viewpoint of Soil Conservation 249

volcanoes. f) Cross-sectional leveling of the bed at intake

0 In the basin with non-active volcanoes, point (BWS. 0) ; five times from the end of there is also a great deal of soil loss because of 1980 to the beginning of 1981.

lumbering, mountain cultivation (including fire In c), each is about 150 ppm and shows nc

agriculture) and river-side erosion. significant difference. But if we compare a)

(3) There are places where river beds are above with b) in a long view, the results indicate the ground at the foot of the mountain. accumulation between the two points whose

(4) The slopes of the main rivers are very distance is 23 km. According to d), SS in the delicate as they meander far upstream. There secondary canals indicates 10 to 30 ppm which

are sometimes natural flood storages at the con- suggests that accumulation occurs immediately fluence of rivers. after the diversion works. This fact varies ac-

(5) Accumulation surpasses the river at the cording to the amount of dredging soil. Since mouth where the river canal flows into the sea there is a lack of data for material e), it is and closes its mouth, and changes the canal impossible to transfer it into the form of inten-

repeatedly. sity of accumulation. About the bed section at The erosion rate of the main rivers in Indo- the head of the primary canal (BWS. 0), the nesia have been tabled by the National Institute term was too short to learn the increase in

of Water Engineering, which corresponds to quantity of accumulation during observation. As Hamamori's data. The erosion rate of Java in the surveys were made in the wet season, they

which there are many active volcanoes is ex- suggest a remarkable change in bed sectien by tremely high, while that of Sekampung is 0.87 the huge discharge of heavy rain.

which is near the world average of 0.7. Lam- (2) Characteristics of rainfall from the view-

pung, whose soil is mainly composed of podzol point of soil erosion and latozol, is a delicate area in terms of erosion. The rivers flowing through Lampung carry a

The Kavangkates dam, on the Brantas river in lot of soil particles in suspension, which finally West Java, was constructed to accomodate de- enter the canals as sediments, and close water

posits of 510, 000 m/year, but actually the sedi- passage. Rainfall is the first agent to produce mentation speed is twice as high causing a soil erosion which makes river water turbid.

serious problem. In Lampung, the problem is Therefore, the characteristics of rainfall should not so severe, but DPU and UNILA are proce- be studied.

eding with the conservation project for Ralem The average soil loss per year is calculated dam. from equation (1) which has been commonly (c) Sedimentation on irrigation canal bed used in the United States since 1950. The Bandarjaya branch of the DPU which A=R•EK•ESL•EC•EP ...... (1) manages the Way Seputih canal system has Where A : Average soil loss per year (ton/

conducted the following observations of sand ha/year)

sedimentation in irrigation canals. R : Erosive potential rainfall factor

a) Daily data of SS at the intake point (BWS. (m2 • ton/ha/hr) 0) ; 1976 - present. K : Soil erodibility factor (hr/m2) b) Daily data of SS at the point 23 km down- SL : Topographical factor concerning stream from the intake point (BWS. 10) ; 1976 angle and length of the slope -1978 . C : Cropping management factor c) Simultaneous observation of SS at the point P : Conservation practice factor 0 km from the intake (BWS.0), down 12 km additionally, R is expressed by equation (2) (BWS. 4) and 39 km (BWS. 25) of a primary R = I30 • (210 + 89 log I) ...... (2) canal ; May 1980 - August 1980. Where Iso : Rainfall maximum for 30 min

d) Six points observation of SS ; November 1979. (mm/30min)

e) Amount of dredging soil from the point of I : Mean rainfall per succession (cm/ 42 km downstream (BWS. 28) to 53 km (BWS. hr) 33) in a primary canal and 15 secondary canals ; The value shown in parenthesis is the total

no measurements. kinetic energy of rainfall per year. 250 熱帯農業27(4)1983

Fig. 4 Rainfall property at Telkbetung between Nov. 1978 and Oct. 1979 (From the data of J. Inoue, Expert of Colombo-Plan)

The above definition of I is not always ac- obtained in Japan. It is also understood from ceptable because rainfall patterns also influence this that most parts of Lampung are suffering soil loss. We could not obtain data on rainfall from the danger of soil erosion. in Lampung, although it should have been pro- Soil loss research in Indonesia is conducted vided. But about two years automatic rainfall at the Erosion Station established in each of record papers had been given by J. Inoue, provinces, e. g., in Lampung, the Metro Erosion Colombo Plan Expert in Indonesia. Then we Station in Metro, which conducts continuous tried to analyze them and got Fig. 4 as the observation. There is also the National Institute result. From the figure it is clear that the of Soil Science in Bogor which overseas the rainfall for 30 min around the peak is closely accumulation and analysis of data. Also the correlated with the total rainfall ; every rainfall universities conduct research using the same pattern has its distinct peak. Then, the defini- procedure as that of the Institute. The procedure, tion of I should be changed from average rainfall in short, is to apply the parameters LISLE for- to the erosive peak, in respect to the estimation mula which is the soil loss prediction formula of A in equation (1). The occurrence of soil in the U. S. A. to the conditions found in Indo- erosion is estimated in Fig. 4 based upon data nesia. The soil vessels of each station were TOMITA et at.: Land Use Strategy in Tropical River Basin from the Viewpoint of Soil Conservation 251 standardized to the American standard which is pung. It shows the land use of the catchment 2 m•~22 m. The quantity of soil loss was obser- area corresponding to each sampling site. ved for each combination of slope, methods of From these data the following results were cultivation, soil and plant species. The primary obtained: data is already published and subsequent data (a) The turbidity (SS) of around 10 ppm agrees is now being analyzed. While the LISLE meth- approximately with that obtained at Way Sepu- od is to treat soil loss of an area of developed tih, though sampling was made in a later period farm land, in Japan, significant soil loss tends of the dry season when the river was in a to occur in the form of erosion or slop destruc- stable stream condition. (b) The area of upland tion by heavy rain immediately after land rec- clearing is not always proportional to the degree lamation. Researches of such cases are now of turbidity. (c) The degree of turbidity does proceeding in Japan. Because the amount and not seem to increase along the downward flow pattern of rainfall of Indonesia is similar to that of a river. This fact may result from the puri- in Japan, soil erosion is also thought to be similar. fication effects in the paddy field where sus- From this viewpoint, soil loss researches must pended solids are precipitated under waterlogged be proceeded and estimating method should be conditions. improved further. These conclusions give a great deal of informa-

(3) The relation of basin coodition and land tion about basin condition and land use but use as compared with other basins more detailed studies are required. To obtain an index of basin condition, we The relationship between the water turbidity measured the amount of suspended soils (SS), and the ratio of the dry field reclamation was at basin of Way Seputih. Then we did further not always linear. SS does not increase in the water analysis in order to compare with other distance downstream. We suppose that this rea- basins relating to the land use. Fig. 5 shows son is due to the effect of the clariffication of sampling sites and their catchment areas. turbid water by sedimentation in the paddy Table 1 shows the results obtained in the fields during flood irrigation. The structure of analysis carried out by the University of Lam- the watershed management model (Fig. 6) which

Fig. 5 Sampling sites for turbidity measurements and catchment areas 252 熱帯農業27(4)1983

Fig. 6 System change and management problem with irrigational development in a paddy field in a tropical region

we tried to make is based on this consideration. of Way Seputih. It shows that the clarif icative effect of the paddy (1) Structure of the model field is significant. In this attempt, fundamentally Sugawara's 3. A Model for Watershed Management : A tank model was adopted for a model which Tank Model transforms daily rainfall into runoff discharge In last section, the existence of the close re- in the river. Then SS producing and changing lation between river water SS and catchment function was introduced into it. land use pattern or condition was concluded. (a) Modelling of runoff This means that if there is some model to sim- It is well known that Sugawara's model which ulate the above relation we can consider the consists of a series of four tanks shows good optimum land use for watershed conservation, agreement with runoff mechanism in Japanese using these model. From this viewpoint, we rivers. When rainfall is put in and evapotran- tyied to construct a watershed management mo- spiration it is taken off from the first tank (if del by means of improvement of Sugawara's sctorage is less than evapotranspiration, it is tank model. Then it was adopted for the basin taken off from the lower tank), we obtain output TOMITA et al.: Land Use Strategy in Tropical River Basin from the Viewpoint of Soil Conservation 253

from the side orifices of four tanks as surface flow, interf low, short term base flow and long term base flow, respectively. Output from the bottom of the tank means permeation. This type of model, however, cannot represent the runoff mechanism in tropical monsoon areas where dry and wet seasons distinctly occur in turn. For these types of areas, more complicated tank model is proposed and is expected to sim- ulate the water movement in the soil. This plain model seems just like the Stanford model. These approach with Stanford model was done by Indonesian scientist too.5) But this model was developed mainly for semiarid area, and not so suitable for monsoon Asia which have rainy season. And it seems too complex to improve for watershed management model. Dr. Sugawara is suggesting a more complicated tank model in his papers, but the hydrological data we can use are not accurate enough for it, so the first type model is adopted here. In the upper tank, primary and secondary soil moisture can change according to their con- tents, so that the later type model can easily express seasonal hydrological change. Evapotran- spiration must be taken off from the first tank, including primary soil moisture. Except for these points, the method of calculation is the same as that of the first type model. Coefficients of the orifices (their size and height) are decided by the trial and error method. (b) Tank models for land use Runoff discharge, washing loads in river beds or suspended solids in flow at the study point will be greatly affected by the way of land use and management of the basin. We intended to deal with some different tank models for each type of land use paddy field, dry field, and forest. A tank model for paddy fields and one for dry fields were prepared similar to the above mentioned tank model. Parameters described in the Fig. 7 were based on standard values used in Japan.6) The model for the forest Fig. 8 Structure of unit block area was supposed to be the same as the one Note : every sign means both water discharge and SS described in the previous section. (c) Structure of the unit block model and syn- paddy field and dry field. Every land category thesis of it for the basin has water balance and SS balance. We express The river basin is divided into unit river this water balance by the unit river basin tank basins based on conditions of river flow, topo- model shown in Fig. 8. Here, for example , Q1 graphy, irrigation and land use. Every unit river means the primary irrigation canal from Way basin consists of some land categories ; mountain, Seputih. Each units were combined linearly to 254 熱帯農業27(4)1983

make basin structure. pography, structure of drain, etc., and was too complex to measure. But for the purposes of (2) SS producing and changing functions There are five main mechanisms which control this paper, the following parametric model is soil loss and river suspension in a river basin. enough to determine reduction retio, i) Producing function of soil loss in dry fields QMT(S) =ƒÁ• QMT1 (S) ...... (6) and mountains, ii) Trapping function from the The starting value of r was 0.5. producing point to river, iii) Sedimentation and (c) Sedimentation and rolling up function in rolling up function in river, iv) Sedimentation the river function in canal, v) Sedimentation function in In the river when the flow velocity is low it

causes sedimentation. When the velocity is high, paddy field. We considered every item as follows : the bottom layer rolls up. But both of these (a) Producing function of soil loss According to Fig. 8, soil loss is produced balance out in a long range, so that we can with outflow from the side orifice of the upper assume balanced flow QQL (B) for balance ve- most tank (in mountain tank QMT1, in dry locity. field tank QFO1). We express it by density, Therefore, the following parametric model was

SS (ppm). used.

SS(ppm)=QMT1(S) or QFO1(S)/ SSa=(QQL/QQL(B)) E•SSb ...... (7)

QMT1(W) or QFO1(W) ...... (3) where, SSb means SS before this function, where, W means water discharge, S means SSa means SS after this function. As the mass suspended SS. of rolling up is never beyond the mass of the

According to LISLE (see Equation (1), (2)) bottom layer, the bottom layer is defined below.

QMT1(S)/area of land category =A...... (4) IF BED<(SSa-SSb) •QQL

As the object of this formula is to estimate SSa = (SSb + BED) /QQL the total soil loss in one year, it is impossible BED =0.0 to estimate soil loss in one day. But there are IF SSa

some examples which research the interaction BED = BED + (SSb - SSa) • QQL

between R(EI) of USLE and A for one con- The initial value of QQL(B) is the median

tinuous rainfall.7) This research shows that it is value in changing the range of SS time-series

expressed by an index function. The interaction data (solid line in Fig. 9). initial value of e was

between daily rainfall and R (EI) has a statisti- 0.5-1. 5.

cal tendency for each location. Therefore every (b) Sedimentation function in canals.

daily rainfall corresponds to R(EI) by the imi- From the observation of SS in irrigation canals

tational generation of randam number with bur- of Way Seputih, the downf low causes sedimen- den which is in proportion to this tendency. In tation and the less its velocity is, the greater

LISLE, C and P are constants between 0, 0 and the sedimentation becomes, therefore

1.0. As the values change according to the QP(S) =•¬Q/Qamximum in design • QI(S) ...... (8)

estimated calculation period, we can not utilize The initial value of c should be set at 1, 0-

the accumulation of the calculated values in 0.4 considering the length of canal from obser-

Indonesia. vation.

The quantity of soil loss which corresponds (e) Sedimentation function in a paddy field

to the daily rainfall data can be predicted by This is assumed to be inversely proportional

the above relation, so the parametric expression to surface velocity, i. e., irrigation rate and depth

for the tank model is, of flooding water from the stoke's law and to

QMT1(S)=ƒ¿•ERƒÀ ...... (5) be directly proportional to the passing paddy

where, R must be calculated from (2) by the field area which is the same as the length of

imitational generation of random number. The canal.

initial value of simulation, QP01(S)=ƒÅ•EQP/QPmax•ED/Dmax •EQP(S) ....(9) for dry field tank : ƒ¿=0.5-0.9, ƒÀ=1.1-1.6

for mountain tank : ƒ¿= 0.1-0.4, ƒÀ= 0.5-1.0 where, D means depth of flooding water, i is a constant which is decided by the passing (b) Trapping function in a block

The trapping function varies according to to- paddy field area (0-1. 0). The initial value of ToMITA et al.: Land Use Strategy in Tropical River Basin from the Viewpoint of Soil Conservation 255

ƒÅ was 0.3 in a large plot to plot irrigation area, observation point (catchment area, 75 km2): 0.5 in small one and 0.8 in irrigation and 1/Jan/1973 - 31/Dec/1975

drainage separated area according to the research Seputih river Ajibaru 038 flow discharge ob- in Japan and Indonesia . servation point (catchment area, 493 km2) : 1

(3) Adaptation of the model to Way Seputih /Jan/1973 - 31/Dec/1975 river basin Seputih river Segalamider flow discharge ob- We tried to adapt this model to Way Seputih servation point where dam construction is river basin (upper area of Rumbia) . River planned (catchment area, 175 km2) basin is divided into four blocks at Sugalamidar, Evapotranspiration data : Long term data evap- Ajibaru and Gunugsugih. Here , otranspiration by Penman's method at Gunung a) Sugalamidar is the proposed dam site . Megang (E. L. 550 m) are as follows :

b) Ajibaru is the intake point of the primary Jan. 2.7 mm/day May 4.1 Sep. 3.3 canal. Feb. 2.7 Jun. 3 .9 Oct. 3.5 c) The south side of the river from Ajibaru to Mar. 3.2 Jul. 2.8 Nov. 3 .3 Gunungsugih is mostly a dry field area . Apr. 4.3 Aug. 3 .2 Dec. 2.8 Block I includes a comparatively steep sloped Way Seputih river SS data, Ajibaru point : area. Block III is, on the whole , flat, and paddy 16/Nov/1976 -1/Oct./1980 field is prevalent in block IV. The land use (b) Result of flow analysis of Sumber Sari pattern in every block is shown in Table 1. In Point block IV, the south side of Way Seputih is Input rainfall is defined by the the next for- irrigated area from Way Sekampung, and not mula with considering area rainfall. from Way Seputih. On the other hand , 60 % Input rainfall= (A24)•~0.3 + (R018)•~0 .3 + of the irrigation water which is taken from (R105)•~0.05 + (R205)•~0.05 Ajibaru and delivered through Way Seputih When there are some points where data are

primary canal is used in the north side of the lacking, we simply used another point' data . By canal, i. e., Way Pungbuan. The acreage of both repeating the traial and error calculation , we area are the same and both Way Seputih and got a adequate parameters of the model. Way Sekampung irrigation systems are control- (c) Result of flow analysis of Ajibaru point led and managed by DPU. We assumed that The definition of input rainfall is the same irrigation water taken from Ajibaru and delive- as (b). After the trial and error calculation we red through Way Seputih primary canal is used got adequate parameters of the model shown in in the whole area of block IV. Fig. 8. The calculated flow discharge (...... ) (a) Data used and the observed flow discharge (•\) are Seputih river basin, A24 rainfall observation shown in Fig. 9.

point:19/Jun/1974 - 17/Nov/1975, 1/Jan/1979 (d) Adaptation of these models to Segalamider - 31/Dec/1980 (Lack of data:4/Mar/1979 - point 26/May/1979 and 29/Jun/1979 -16/Oct/1981) We have the flow discharge data from 1979 Seputih river basin, R018 rainfall observa- to 1980, but compared with the total rainfall , the tion point : 1/Jan/1973 - 31/Dec/1975 (Lack total flow discharge is too much . For example, of data 1/Dec/1973 - 31/Dec/1973 ,1/Jan/1979 in 1980 the total rainfall was 2 , 630 mm, estima- - 30/Nov/1980) ted evaporation 1,210 mm and observed flow Seputih river basin, R137 rainfall observation discharge 1, 730 mm (with catchment area , 175 point : 25/Jun/1974 - 31/Dec/1975, 1/Jan/1979 km2). It was impossible for the calculated value - 31/Dec/1980 from this model to correspond with the observed Metro R105 rainfall observation point : 1/Oct data on teh hydrograph. As there is not much /1973 - 31/Dec/1975,1/Jan/1979 - 31/Dec/1980 difference between the model of Sumber Sari

(Lack of data : Apr/1980, Sep/1980) point and that of Ajibaru point, we adopted the Kotabumi R205 rainfall observation point : 1/ same model at the middle point of the two . Jan/1973 - 31/Dec/1975, 1/Jan/1979 - 31/Dec/ We adopted the tank model of Ajibaru, calcula- 1981 ted the flow discharge and compared it with Seputih river Sumber Sari 037 flow discharge the observed hydrograph at Segalamidar. On the 256 熱帯農業27(4)1983

Fig. 9 Comparison of calculated SS values with observed ones at Ajibaru (1979 -1980)

whole, as mentioned above, the calculated value under high temperatures and humidity, (ii) Low is lower, but the pattern of the hydrograph is soil fertility which is caused by (i), (iii) High similar. We can use the tank model of Ajibaru soil loss rate under local and heavy precipitation. for a similar river basin. Under these conditions the most reasonable crop is rice cultivation in paddy field, which (e) Result of SS of Ajibaru point We used the river basin management model many researchers have already pointed out. In which is shown in section 2 and defined the this paper we confirm this, by simulation model and learned quantitatively that lost soil is trapped parameters of soil loss and SS after repeating the simulation calculation with the initial value in paddy fields by irrigation water. It means shown in section 2 (2). After the process men that the same trap function operates on soil tioned above, we got the calculated values at fertility, In tropical zone where soil loss can not the Ajibaru point. They are shown in Fig. 9 be avoided more or less, the rice cultivation in as well as the values of parameters. a paddy filed is one of the agricultural way to

In order to consider the influence of the dif- highly utilize the river basin resources. From ferent land use pattern in every block upon this point of view it is very desiable to extend the behavior of SS in the whole river, we the area of the paddy field by the construction calculated and compared the values of SS at of an irrigation canal, which has been done in the lowest point of every block. This is accom- many places. And, in order to maintain the stability of these facilities some methods should panied by the calculated value of the Rumbia be taken to check sedimentation in canals and point which comes from the assumption that there is no outflow from block III to the main river control the flow capacity. because there is no dry field in block III. In For example

this case Q and SS at the Rumbia point mainly a) To develop a soil loss prevention method of

correspond with outflow from the paddy field agriculture in the dry field area in the river

area, which contains the return flow. We could basin. Among USLE parameters, R and K are

understand the clarification, efficiency of paddy decided by natural conditions and can not be

field on the level of the whole river basin. artificially controlled, but SL can be controlled

4. Agricultural Land Use in Consideration by political measure, i, e., restriction prohibition

of Watershed Conservation•cinstead of con- on the dry cultivation, on definite slope area at

elusion least with plowing. In Way Relem dam catch-

The characteristics of the ecological environ- ment area the government of Lampung chose

ment of agriculture in tropical region are : (i) this kind of agricultural politics. But in Indo-

High rate of decomposition of organic matter nesia complex cultivation is prevalent and the TOMITA et al.: Land Use Strategy in Tropical River Basin from the Viewpoint of Soil Conservation 257 high starch producing capacity of cassava and system now impedes the introduction of agricul- maize is very important. We must develop the tural machines, extension of modern agriculture technology of the C and P factors to encourage and effective water control for rotation from the cultivation of maize and develop a new paddy to dry field. A new plot-to-plot system planting system to minimize the fallow field should be studied retaining its advantages and period. inproving above-mentioned defects under the b) In order to decrease the damage of canal by development of modern technology. lost soils and to increase fertility of the paddy field efficiently, a land use plan is important. References From this point of view a traditional land recla- 1. KAIDA, Y. 1980 Physiographic Regions in the mation system like IIIb type should be reconsid- Komering-Ogan River Basin, South Sumatra, ered under the condition of modern technology South Sumatra Man and Agriculture, pp. 1-19, of civil engineering, rather than a large scale The Center for Southeast Asian Studies of Kyoto Univ. canal system. 2. Statistik Indonesia 1978/1979, Biro Pusto Statistik, c) In the area where there is shortage of water Jakarta. resources under natural conditions, it is neces- 3. Scrimshaw, N. S., Taylor and Food, L. 1980 Sci sary for them to employ a large scale canal entific American, Japan Edition vol. 10-11, p. 43. system. In these area, a land use plan should 4. Report on activities of project ATA 105 1976, be considered to decrease the damage of sedi- 1977 soil physics. Soil research Institute. Bogor. mentation, including such plans as prohibiting 5. Analysis and simulation of upper Way Seputih dry cultivation in a catchment area, developing Watershed, Study of Agro-Ecosystem in the IIIa,b-type paddy field in a dam catchment area Framework of Watershed Management, Book I, and lead clean water through paddy field clari- Center for National Resource Management and fication efficiency to a dam reservoir. Environmental Studies, Bogor Agricultural Univ., 1981. d) The paddy field should have large average 6. MARUYAMA,T., TOMITA, M., KOBAYASHI,S. 1979 for flooding water from the point of clarification, Water balance analysis of large area with complex deficiency and effective utilization of inflow soils tank model, Trans. JSIDRE. vol. 47, p. 190. and fertilizer. The plot to plot irrigation system 7. HOSOYAMADA,K. 1980 Mass of soil loss on is prevalent in Indonesia as a method of irriga- andosol, Research of field conservation. No. 1 tion and effective utilization of water resources. : p. 26. But it is a fact that the plot-to-plot irrigation 258 熱帯農業27(4)1983

摘要

インドネシア国ランポン州における流域管理について

土壌保全の見地からの熱帯流域土地利用の合理的あり方

冨田正彦*・豊田勝**・竹中肇*・鈴木光剛***

K.E.S.マニク***・B.ロサジ****

*東京大学農学部113東京都文京区弥生 **新潟大学農学部950-21新潟市五十嵐二の町 ***筑波大学農林工学系305茨城県新治郡桜村 ****ランポン大学農学部インドネシア共和国ランポン州

インドネシア国ランポン州におけるめざましい水田農生態系の安定的確立,とくにP欠乏に対応した地域的物業の発展は大規模灌漑施設の建設による地域生態系の人質循環系やそれに適合する水稲品種の開発,b)流域内為的改変によるところが大きいしかしこうした人為的改畑地帯における土壌流亡防止農法の開発,これには裸地変は,地域の自然と社会との関係になじみの薄いもので期間を最小に抑える作付体系の開発なども含まれる, あったこともあって,ランポン州の農業は必ずしも安定 c)畑地から流亡土の灌漑水路への沈積も抑え,水田へ的に確立されているとはいい難く,種々の災害をも誘発の地力補給に寄与しうるためには,現行の大用水路建設している. 型開田もさることながら,畑地流出水が隣接水田に流入そこで本論ではまずランポン州の農業生態系と,流域するような土地利用計画上の工夫も望まれる.d)流入スケールでのその荒廃,とくに土壌侵食とその流域的影懸濁灌漑水・地力成分を有効利用しつつ流域水系の水質響を考察した上で,流域管理のためのシミュレーション保全に寄与しうるには末端水田は湛水面積の広いことがモデルの開発を試み,これをセプティ川流域に適用して望しく,現行の田越し灌漑はこの点で優れている.しか検討した. しこれが農業機械導入,水管理操作の困難さを介して地その結果地域農業生態系とその基盤諸施設の安定的維域農業の足かせの一つになっていることも否めない.し持には以下のような方策の必要なことが明らかにされた. たがって利点を保持しつつ,難点を解消しうる,近代土 a)ポドゾル土壌の貧栄養性を考慮した熱帯型人工水田木技術の枠組のもとでの新しい型式の田越し灌漑方式が 1982年12月22日受理開発されるべきである.

Masahiko TOMITA**, Masaru TOYOTA***, Hajime TAKENAKA**, Mitsukata Suzuki****, K

Masahiko TOMITA, Masaru TOYOTA*, Hajime TAKENAKA, Mitsukata Suzuki**, K