# **_4. Brantas Basin Water Management Issues_**

# **_4.1 Physical Characteristics of the Brantas Basin_**

The Brantas river basin is located within the province of East Java in Indonesia (see Figure 1). It has an area of approximately 11,800 km<sup>2</sup> and makes up 24.6% of East Java’s land area. The basin is bounded by Mt. Bromo (2,393m) and Mt. Semeru (3,676m) on the east, a series of low ridges to the south, Mt. Wilis (2,169m) and its ridges on the west, and the Kedung low ridges and the Madura Strait on the north. The Brantas River is 320 km long, with its headwaters located in the Arjuno volcanic massif, a substantial topographic feature in the southeast portion of the basin. (Ramu 2004 ) The river flows around the massif clockwise, through the Malang Plateau, then west through the major dam and reservoir complex, made up of Sengguruh, Sutami/Lahor, Wlingi, and Lodoyo. At the southwestern portion of the basin, where the Ngrowo River joins it, the Brantas turns north towards the agricultural plains region, and then east through the delta regulated by the New Lengkong Barrage. This structure divides the Brantas into the Porong and Surabaya Rivers and the Porong and Mangetan Canals. The canals provide irrigation water for the paddy-growing region before discharging into the Madura Strait. The Porong River serves as flood diversion canal during the rainy season, and the Surabaya River is a primary source of raw water for Surabaya City. The channel slopes of the upper Brantas basin are relatively steep (greater than .005), becoming more gradual lower in the system (.001 or less). (IFPRI 2002) There are two active volcanoes within the basin. Mt. Semuru erupts frequently with most of its debris depositing outside of the basin. The eruptions of Mt. Kelud, on an average cycle of 15 years, have had great effect upon river morphology and have led to considerable loss of human life and property damage. Its erupted material amounts to about 200 million m<sup>3</sup> . This volcanic ash has been a primary contributor to soil fertility for the basin, as well as to riverbed aggravation and reservoir sedimentation.

The tropical monsoon climate of the basin comprises the rainy season from November to April and the dry season from May to October. The annual mean temperature in the basin ranges from 24.2 to 26.6 degrees Celsius. Rainfall averages 2,000 mm, with over 80% occurring in the rainy season. Average rainfall in higher elevations is between 3,000 and 4,000 mm. Variation of rainfall from year to year is large: in wet years, rainfall averages 2,960 while in dry years (which occur once every three years on average) it averages 1,370 mm. The yearly relative humidity within the basin ranges from 75% to 82%.

The average surface water potential in the Brantas basin is estimated to be approximately 12 billion m3, with the average flow at an estimated 3 billion m3, or 25% of available surface water. The average annual flow is 823 m<sup>3</sup> /s in the upstream reaches, 3,859 m<sup>3</sup> /s in the midstream, and 5,300 m<sup>3</sup> /s in the downstream area. Groundwater exists mainly in the regencies of Madura, Surabaya, and Madiun. In the basin, 447 deep wells irrigate 25,730 ha. There are 38 deep wells for raw water, supplying large municipalities with a significant portion of their supply. Rural water supply is mainly from nearly 27,600 shallow wells. (Ramu 2004) 8