View of Determination of Curve Number for the Temef Watershed, Timor Tengah Selatan Regency

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*Corresponding author: Water Resources Engineering Department, Faculty of Engineering, Universitas Brawijaya, 65145, Indonesia E-mail address: [email protected] (D. F. B. Welkis)

doi: https://doi.org/10.21776/ub.pengairan.2023.014.02.1

Received: 2022-09-12; Revised: 2022-11-14; Accepted: 2023-02-10.

Vol. 14 No. 02 (2023)

Jurnal Teknik Pengairan: Journal of Water Resources Engineering

Journal homepage: https://jurnalpengairan.ub.ac.id/index.php/jtp

Original research article

Determination of Curve Number for the Temef Watershed, Timor Tengah Selatan Regency

Davianto F. B. Welkis*

^a

, Donny Harisuseno

^a

, Sri Wahyuni

^a

, Sebrian Mirdeklis Beselly

^a,b,c

aWater Resources Engineering Department, Faculty of Engineering, Universitas Brawijaya, 65145, Indonesia

bDepartment of Coastal & Urban Risk & Resilience, Coastal System & Engineering and Port Development, IHE Delft Institute for Water Education, 2611 AX, The Netherlands

cDepartment of Hydraulic Engineering, Section Coastal Engineering, Delft University of Technology, 2628 CN, The Netherlands

A R T I C L E I N F O A B S T R A C T Keywords:

Antecedent moisture condition;

Curve number;

Hydrologic soil group

This study aims to show that the Curve Number (CN) method can represent the relationship between rainfall runoff in the Temef Watershed, an area of East Nusa Tenggara. The method used in this study was quantitative analysis by linking watershed characteristics such as soil, vegetation, and land use with the CN curve number, which shows the potential flow for a certain rainfall. CN data were re-analyzed using a computer program to obtain actual field conditions, which were then classified to obtain land cover quality, soil type, CN value, and Antecedent Moisture Condition (AMC) value for the Temef Watershed. Based on the study's results, it was found that the Temef Watershed was dominated by secondary dry land forest cover, which ranged from 28.50-52.00%. The lithological texture of rocks in the Temef Watershed is dominated by conglomerate and gravel with a gradation rate from medium to high; sandy marl, sandstone, tuft, and dacite of medium to a high gradation; and scaly clay with very low to low gradation. The CN value is 69.45, classified as medium gradation at normal soil moisture levels. The benefits of this result include the practical application of the CN method, the provision of accurate field conditions through re-analysis, insights into dominant land cover and lithological texture, and the establishment of CN values for effective water resource management.

1. Introduction

The Province of East Nusa Tenggara (Nusa Tenggara Timur, NTT) is a dry region with a dry season that occurs relatively longer than the rainy season [1]. The mountainous topography and sparse vegetation in an area with a relatively short wet season lead to relatively small rainfall, with an average of 1000 mm/year [2]. Because the rainfall in NTT is relatively small, this region often experiences problems with water availability [3]. Regarding the flow condition of the Temef River, during the rainy season, the discharge is extremely large, while in the dry season, the discharge of the Temef River decreases drastically [4]. Problems of water availability in NTT, specifically in Timor Tengah Selatan (TTS) Regency, may be resolved by constructing water catchment structures such as reservoirs and dams through a stage of planning or development of water resources. One of the hydrologic variables that is very important in the planning or development of water resources is surface flow or runoff

.

Surface runoff that occurs in the Temef River also contributes to occurrences of flooding in the downstream part of the Benanain Watershed, which occurs almost every year and even occurs multiple times in a year and has a tendency to increase in greater periods and frequency depending on the rainfall intensity that occurs in the upstream part of Benanain Watershed. Therefore, an investigation of soil type and land use needs to be conducted, and thus, analysis is required regarding the influence of land use and soil type on the flow curve, the Curve Number (CN), as one of the determining variables for the change in discharge in the Temef Watershed.

It is one of the methods developed by the Soil Conservation Service (SCS). It represents a function of watershed characteristics such as soil type, cover plants, land use, moisture, and soil tilling methods.

Other researchers had previously researched the Curve Number (CN) value. [5] researched determining the Curve Number (CN) value for the watershed with an oval shape using the HEC-HMS model. Some of the influencing

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104 parameters include the topography of the watershed, land use, soil type, and soil moisture. The HEC-HMS model simplified factors such as soil characteristics, geological formation, and land use, called the CN factor. Research has also been conducted to determine the Hydrologic Soil Group (HSG) in calculating flood discharge in the Brantas Hulu Watershed [6]. The HSG that were obtained from the Harmonized World Soil Database (HWSD) map for the Brantas Hulu Watershed were D (clay), B (silty clay), and A (sandy clay). The determination of HSG from the HWSD soil map used the water loss method of NRCS-CN (Natural Resources Conservation Soil-Curve Number) and NRCS hydrograph units; the best calibration was found from RMSE and differing heights for AMC II and λ = 0.2 for March 2007 (RMSE = 0.55) and AMC II and λ = 0.05 for December 2007 (RMSE = 0.65).

Research in the Temef Watershed for the CN value has been previously conducted, even though it was limited to an overview of the hydrologic soil group in the Temef Watershed. The analysis results indicated a level 3 permeability type with a lithology of limestone, dense volcanic breccia, sandy marl, conglomerate, and alluvium composed of sand and gravel with a high to medium permeability. Metamorphic and basal rocks have a medium to low permeability, and scaly clay has a low to very low permeability [4].

This research aims to determine the Curve Number value based on a hydrogeologic map converted to a Hydrologic Soil Group map for the Temef Watershed in Timor Tengah Selatan (TTS) Regency. This research will provide information and solutions for controlling floods in the Temef Watershed.

2. Method

2.1. Research Location

The research was conducted at the location of the Temef Watershed, which is located on the Temef River, Konbaki

Village in Polen Sub-District and Oenino Village in Oenino Sub-District, Timor Tengah Selatan Regency, Province of East Nusa Tenggara. Geographically, the Temef Watershed is located within the coordinates of 9° 26' – 10° 10" South Latitude and 124° 49' 01" – 124° 04' 00" East Longitude. TTS Regency has a tropical climate, as with other regions in the Province of Nusa Tenggara Timur, with a temperature of 27- 29 °C and an average annual rainfall of approximately 1446.4 mm/year [7]. The location of the Temef Watershed can be seen in Figure 1.

The area of the Temef Watershed is 550.98 km^2, with a primary river length of 45.35 km. Temef Watershed has five sub-watersheds: the Bijeli Sub-watershed, Besi Sub- watershed, Laku Sub-watershed, Noelnoni Sub-watershed, and Benanain Sub-watershed.

2.2. Research Data

This research required supporting data for determining the Curve Number (CN) value. Primary data was collected in the form of documentation and measuring the flow elevation of the Temef River, which the researcher conducted.

Secondary data consisted of maps of land use, geology, hydrogeology, geography, administration, and data on watershed characteristics. The secondary data above were obtained from the Nusa Tenggara II Watershed Authority, the Department of Public Works of the Province of NTT, and the Regional Development Planning Agency of TTS Regency.

2.3. Research Method

In this research, data processing was conducted according to a precise and appropriate methodology. The method that was utilized in this research was the Curve Number (CN) method. For the calculation of the Curve Number (CN) value for a watershed that possesses different kinds of soil types and ground cover used equation 1 [8].

Figure 1. Location of Temef Watershed

WATER RESOURCES MANAGEMENT WATER RESOURCES ENGINEERING FACULTY OF ENGINEERING

BRAWIJAYA UNIVERSITY

LEGEND

Dischage Posts Rain Posts

Temef River Category

Tributrary Tributrary Main River Temef River East Nusa Tenggara Province REGENCY

BELU REGENCY KUPANG REGENCY SOUTH CENTRAL TIMOR REGENCY NORTH CENTRAL TIMOR REGENCY Temef Discharge Post

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105

Table 1. Limit of seasonal rainfall for AMC used in SCS-CN [9]

Antecedent Moisture Condition (AMC)

Total rainfall of the previous five days (mm)

Dry Season Planting Season

I <12.7 < 35.6

II 12.7 – 27.9 35.6 – 53.3

III > 27.9 > 53.3

Figure 2. Ground cover map for Temef Watershed

CN

_comp

=

^(CN¹^{. A}¹^)+(CN²^{. A}²^)+…+(CNⁿ^{. A}ⁿ⁾

∑ A

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Remarks, CN_comp is composite CN, CN1 is curve number for soil type and ground cover 1, A1 is a combined area of soil type and ground cover 1 (km²), and ∑A is a total combined area of soil type and ground cover (km²).

The obtained CN value for various kinds of land use in different conditions is related to the Antecedent Moisture Condition (AMC). Antecedent Moisture Condition is a condition of previous moisture present in the soil type. AMC is grouped into three classes based on the season: AMC I for the dry season, AMC II for the transitional season, and AMC III for the very wet season [9]. AMC classes may be determined according to the limit of seasonal rainfall based on Table 1.

Table 1 shows that the Antecedent Moisture Condition (AMC) value is divided into three parts covering different rainfall and seasonal conditions. The AMC value can be calculated with an equation; for AMC I or AMC III, the equivalent CN value can be calculated with the following equation [10]:

CN(I) =

4,2 CN (II)

10 - 0.058 CN (II)

(2) CN(III) =

23 CN (II)

10 + 0.13 CN (II)

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Where CN (I) is CN for Dry Conditions, CN (II) is CN for Normal Conditions, and CN (III) is CN for Wet Conditions.

3. Results and Discussion 3.1. Analysis of Curve Number (CN)

The CN method is an empirical approach used to calculate direct runoff from rainfall occurrences, starting from the rainfall catchment area (watershed) [11]. The CN method estimates the runoff from the relationships among rainfall, ground cover, and hydrologic soil group (cover complex classification) [12]. The CN value may be estimated if the watershed's soil classification and ground cover are known.

Determining the CN value must also consider the conditions of previous moisture in the soil or the Antecedent Moisture Conditions (AMC). The soil in the saturated condition will contribute to a large runoff, and soil in the dry condition will contribute little to runoff [12].

3.1.1. Classification of Ground Cover

Results of the analysis of ground cover for the Temef Watershed led to obtaining data that indicated that the Temef Watershed has seven kinds of ground cover. The kinds of ground cover are airport, primary dry land forest, secondary dry land forest, dry land farming and undergrowth, savanna,

Ground Cover Map for Temef Watershed

LEGEND River

Temef Sub-Watershed

Primary Dryland Forest Secondary Dryland Forest Settlement Areas Dryland Farming and Bush Savanna

Shrubs Bare Ground

W 60

W 70 *

* W 100

R 10

R 30 W 80

W 90

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106 brushwood, and open ground. The following is the map of ground cover in the Temef Watershed, depicted in Figure 2.

3.1.2.Classification of Hydrologic Soil Group (HSG)

Soil type

affects

the rainfall; thus, the soil is divided into several classes. The division of soil classes is based on the hydrologic aspect, considered from the infiltration rate. A greater gradation of soil grains also means a greater rate of infiltration. Conversely, soil with a finer grain will have a lower rate of infiltration, which causes surface runoff.

Classification of hydrologic soil groups is based on the soil map, which contains similar properties such as depth of layers or depth of groundwater surface, rate of water transmission, texture, and structure, as well as the constructed water level height when the saturated condition is achieved, resulting in similar runoff [13]. Based on Figure 3, the hydrologic soil groups of the Temef Watershed are divided into nine kinds.

The map makes it possible to find the lithology, area, gradation, and percentage of each existing soil type.

Based on the hydrogeologic map of the Temef Watershed in Figure 3, hydrologic soil groups of the Temef Watershed are divided into nine kinds with gradations of very low to low, low to medium, and medium to high with B, C, and D HSG categories.

Below in Table 2 is the hydrologic soil group matrix of the Temef Watershed, which explains the lithological condition of the watershed.

3.1.3.Calculation of the Curve Number (CN) Value

The runoff curve number is the net value that is obtained after subtraction of the amount of water being absorbed (infiltrate), which is based on the level of soil/rock permeability from the hydrogeologic map and the runoff value from the ground cover classification in the area of the Temef Watershed. The CN value was calculated for each sub- watershed, resulting in a value that approaches the actual value or the value based on the conditions on the field.

Determination of the CN value with the SCS-CN method can be seen in Table 3 for the Benanain Sub-watershed / W80 Sub- watershed, Table 4 for the Noelnoni Sub-watershed / W90 Sub-watershed, Table 5 for the Laku Sub-watershed / W100 Sub-watershed, Table 6 for the Besi Sub-watershed / W70 Sub- watershed, and Table 7 for the Bijeli Sub-watershed / W60 Sub-watershed.

Based on the determination of the HSG from the conditions of aquifers, lithology, and land cover, the CN value was obtained for each sub-basin of the Temef Watershed, as displayed in Tables 3-7. The summary of CN values is shown in Table 8.

Figure 3. Hydrogeologic Map of Temef Watershed

Hydrogeologic Map of Temef Watershed

Lithology and Permeability

Lherzolite and serpentinite basalt (Low to medium permeability)

Coral limestone, local karst (Medium to high permeability)

Hard limestone and calcilutite (Medium to high permeability)

Various kinds of metamorphic rocks from slate to gneiss, amphibolite, quartzite, and granulite (Low to medium permeability) Alluvium, comprised of sand, gravel, pebbles (Medium to high permeability)

Hard volcanic breccia, agglomerate, lava, and tufts (Medium to high permeability) Conglomerate and pebbles, loose in the upper part and compact in the lower part (Medium to high permeability) Scaly clay containing chunks of other rocks (Very low to low permeability) Sandy marl alternating with sandstone, conglomerate, and dacite tufts (Medium to high permeability)

W 60

W 70 *

* W 100

R 10

R 30 W 80

W 90

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107

Table 2. Hydrologic soil group matrix of Temef Watershed [4]

No

Aquifer Legend Color

Lithology Gradation HSG Sketch

1 Light Brown

Hard volcanic breccia, agglomerate,

lava, and tufts Medium to High B

2 Brown

Various kinds of metamorphic rocks from slate to gneiss, amphibolite, quartzite, and granulite

Low to Medium C

3 Brown Lherzolite and serpentinite basalt Low to Medium C

4 Brown Scaly clay containing chunks of other

rocks Very Low to Low D

5 Light

Green Hard limestone and calcilutite Medium to High B 6 Light

Brown

Sandy marl alternating with sandstone, conglomerate, and dacite tufts

Medium to High B

7 Light

Green Coral limestone, local karst Medium to High B

8 Blue

Conglomerate and pebbles, loose in the upper part and compact in the lower part

Medium to High B

9 Light Brown

Alluvium, comprised of sand, gravel,

pebbles Medium to High B

Table 3. Curve Number (CN) for the Benanain Sub-watershed / W80 Sub-watershed

No Lithology Ground Cover HSG A B C D CN

Value

Area

(Km²) Weight CN Weight 1 Coral limestone, local

karst

Secondary Dry

Land Forest B 25 55 70 77 55 2.273 0.114 6.286

2 Coral limestone, local karst

Secondary Dry

Land Forest B 25 55 70 77 55 1.679 0.084 4.644

3 Coral limestone, local

karst Savanna B 30 58 71 78 58 0.608 0.031 1.772

4 Conglomerate and pebbles

Secondary Dry

Land Forest B 25 55 70 77 55 5.659 0.285 15.650

5 Conglomerate and

pebbles Savanna B 30 58 71 78 58 3.410 0.171 9.946

6 Scaly clay Secondary Dry

Land Forest D 25 55 70 77 77 4.960 0.249 19.202

7 Alluvium, comprised of sand, gravel, pebbles

Secondary Dry

Land Forest B 25 55 70 77 55 1.267 0.064 3.502

Secondary Dry

Land Forest B 25 55 70 77 55 0.016 0.001 0.045

Land Forest D 25 55 70 77 77 0.016 0.001 0.062

Total 19.888 1.000 61.160

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108

Table 4. Curve Number (CN) for the Noelnoni Sub-watershed / W90 Sub-watershed

Value

Area

(Km²) Weight CN Weight 1 Coral limestone, local karst Secondary Dry

Land Forest B 25 55 70 77 55 2.086 0.110 6.033

2 Coral limestone, local karst Open Ground B 25 55 70 77 58 0.002 0.000 0.005 3 Coral limestone, local karst Secondary Dry

Land Forest B 30 58 71 78 55 0.278 0.015 0.803

4

Sandy marl alternating with sandstone,

conglomerate, and dacite tufts

Secondary Dry

Land Forest B 25 55 70 77 55 6.779 0.357 19.609

5

Open Ground B 30 58 71 78 58 0.239 0.013 0.730

6 Conglomerate and pebbles Secondary Dry

Land Forest B 25 55 70 77 55 8.844 0.465 25.583

7 Conglomerate and pebbles Open Ground B 25 55 70 77 58 0.373 0.020 1.138 8 Alluvium, comprised of

sand, gravel, pebbles

Secondary Dry

Land Forest B 25 55 70 77 55 0.413 0.022 1.195

Total 19.013 1.000 55.097

Table 5. Curve Number (CN) for the Laku Sub-watershed / W100 Sub-watershed

No Lithology Ground

Cover HSG A B C D CN

Value

Area

(Km²) Weight CN Weight 1 Scaly clay containing

chunks of other rocks Settlement D 79 86 90 92 92 0.002 0.00001 0.001 2 Scaly clay containing

chunks of other rocks Savanna D 30 58 71 78 78 0.054 0.0003 0.022

Secondary Dry Land

Forest

B 25 55 70 77 55 26.690 0.139 7.650

karst Settlement B 79 86 90 92 86 4.386 0.023 1.966

karst Savanna B 30 58 71 78 58 30.108 0.157 9.100

6

Secondary Dry Land

Forest

B 25 55 70 77 55 99.768 0.520 28.595

7

Brushwood B 29 57 70 77 57 12.896 0.067 3.831

8

Settlement B 79 86 90 92 86 0.374 0.002 0.168

9 Sandy marl

alternating with Savanna B 30 58 71 78 58 3.989 0.021 1.206

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109

No Lithology Ground

Cover HSG A B C D CN

Value

Area

(Km²) Weight CN Weight sandstone,

conglomerate, and dacite tufts 10 Conglomerate and

pebbles

Secondary Dry Land

Forest

B 25 55 70 77 55 11.952 0.062 3.426

11 Conglomerate and

pebbles Brushwood B 29 57 70 77 57 1.529 0.008 0.454

12

Alluvium, comprised of sand, gravel, pebbles

Secondary Dry Land

Forest

B 25 55 70 77 55 0.148 0.001 0.042

Total 191.895 1.000 56.463

Table 6. Curve Number (CN) for the Besi Sub-watershed / W70 Sub-watershed

Value

Area

(Km²) Weight CN Weight 1

Hard volcanic breccia, agglomerate, lava, and tufts

Primary Dry Land

Forest B 25 55 70 77 55 19.723 0.157 8.612

2

Secondary Dry

Land Forest B 25 55 70 77 55 1.777 0.014 0.776

3

Dry Land Farming

and Undergrowth B 51 67 76 80 67 0.011 0.000 0.006

4 Scaly clay Primary Dry Land

Forest D 25 55 70 77 77 1.761 0.014 1.077

Land Forest D 25 55 70 77 77 44.384 0.352 27.133

6 Scaly clay Savanna D 30 58 71 78 78 0.115 0.001 0.071

Land Forest D 25 55 70 77 77 0.002 0.000 0.001

Secondary Dry

Land Forest B 25 55 70 77 55 0.867 0.007 0.379

karst Savanna B 30 58 71 78 58 0.001 0.000 0.000

10

Primary Dry Land

Forest B 25 55 70 77 55 1.514 0.012 0.661

11

Secondary Dry

Land Forest B 25 55 70 77 55 29.027 0.230 12.675

12

Open Ground B 30 58 71 78 58 0.183 0.001 0.084

13

Savanna B 30 58 71 78 58 0.025 0.000 0.012

14 Hard limestone and calcilutite

Primary Dry Land

Forest B 25 55 70 77 55 1.414 0.011 0.617

Secondary Dry

Land Forest B 25 55 70 77 55 5.724 0.045 2.499

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110

Value

Area

(Km²) Weight CN Weight 16 Hard limestone and

calcilutite

Dry Land Farming

and Undergrowth B 51 67 76 80 67 0.488 0.004 0.259

17

Secondary Dry

Land Forest B 25 55 70 77 55 12.741 0.101 5.563

18

Open Ground B 30 58 71 78 58 0.052 0.000 0.024

19 Lherzolite and serpentinite basalt

Primary Dry Land

Forest C 25 55 70 77 70 1.441 0.011 0.801

Secondary Dry

Land Forest C 25 55 70 77 70 4.011 0.032 2.229

Dry Land Farming

and Undergrowth C 51 67 76 80 76 0.698 0.006 0.421

Total 125.959 1.000 64.233

Table 7. Curve Number (CN) for the Bijeli Sub-watershed / W60 Sub-watershed

Value

Area

(Km²) Weight CN Weight 1

Primary Dry

Land Forest B 25 55 70 77 55 13.548 0.071 3.878

2

Secondary Dry

Land Forest B 25 55 70 77 55 0.110 0.001 0.031

3

Brushwood B 29 57 70 77 57 0.489 0.003 0.145

4 Scaly clay Primary Dry

Land Forest D 25 55 70 77 77 6.471 0.034 2.593

Land Forest D 25 55 70 77 77 64.679 0.337 25.919

6 Scaly clay Brushwood D 29 57 70 77 77 0.295 0.002 0.118

7 Scaly clay Savanna D 30 58 71 78 78 0.251 0.001 0.102

8

Primary Dry

Land Forest B 25 55 70 77 55 8.211 0.043 2.350

9

Secondary Dry

Land Forest B 25 55 70 77 55 27.723 0.144 7.935

10

Open Ground B 30 58 71 78 58 1.260 0.007 0.380

Primary Dry

Land Forest B 25 55 70 77 55 4.532 0.024 1.297

Secondary Dry

Land Forest B 25 55 70 77 55 17.055 0.089 4.882

13 Hard limestone and

calcilutite Brushwood B 29 57 70 77 57 0.088 0.000 0.026

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111

Table 8. Summary of curve number values for Temef Watershed

No Sub-Watershed Symbol Area (Km²) Watershed Slope InitAbst CN BasinLag S

1 Bijeli W60 192.12 27.25 28.02 66.12 4.47 140.12

2 Besi W70 128.62 22.00 32.14 64.23 4.38 160.69

3 Benanain W80 19.87 14.20 59.88 61.16 2.07 299.39

4 Noelnoni W90 19.01 16.67 76.39 55.10 2.77 381.94

5 Laku W100 191.88 14.07 42.67 56.46 6.49 213.36

Figure 4. Curve Number (CN) grid map of Temef Watershed

Value

Area

(Km²) Weight CN Weight 14 Conglomerate and

pebbles

Secondary Dry

Land Forest B 25 55 70 77 55 10.338 0.054 2.959

15 Conglomerate and

pebbles Open Ground B 30 58 71 78 58 0.376 0.002 0.114

16

Slate to gneiss, amphibolite, quartzite, and granulite

Primary Dry

Land Forest C 25 55 70 77 70 24.399 0.127 8.889

17

Slate to gneiss, amphibolite, quartzite, and granulite

Brushwood C 29 57 70 77 70 0.058 0.000 0.021

Primary Dry

Land Forest C 25 55 70 77 70 3.336 0.017 1.215

Secondary Dry

Land Forest C 25 55 70 77 70 8.516 0.044 3.102

20 Lherzolite and

serpentinite basalt Brushwood C 29 57 70 77 70 0.365 0.002 0.133

21 Lherzolite and

serpentinite basalt Savanna C 30 58 71 78 71 0.044 0.000 0.016

Total 192.144 1.000 66.116

CURVE NUMBER (CN) GRID MAP OF TEMEF WATERSHED

LEGEND River

Temef Sub-Watershed CN Values of Temef Watershed CN Values Range

W 60

R 10

R 30 Benanain Sub WS

Noelnoni Sub WS

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112 The CN value is a crucial parameter in hydrological modeling as it represents the combination of Hydrologic Soil Group (HSG) values and the runoff value based on land cover [14]. The CN value provides valuable insights into an area's potential flow and infiltration characteristics. In the context of the Temef Watershed, the CN values ranging from 55.10 to 66.12, as depicted in Figure 4, indicate a relatively high runoff potential. It implies that the watershed is prone to increased surface runoff, affecting water resource management and flooding events.

According to the research conducted by [15], it is established that larger CN values correspond to lower infiltration rates. Areas with higher CN values, like the Temef Watershed, have a decreased ability to absorb water into the soil. Consequently, a greater percentage of precipitation is turned into surface runoff, resulting in higher volumes of runoff and the possibility of severe floods. Knowing the correlation between CN values and infiltration rates is essential for efficient watershed management and the implementation of suitable mitigation measures to regulate runoff and avoid negative consequences.

The elevated runoff potential suggested by the CN values emphasizes the necessity for proactive watershed management techniques in the Temef Watershed. To address the issue of high CN values, it is recommended to prioritize the implementation of strategies that improve infiltration capacity. This can be achieved by adopting soil conservation practices and land management approaches that boost water retention [16]. Furthermore, this data can improve land-use planning choices by promoting the adoption of strategies that reduce surface runoff, such as protecting native plants and applying sustainable land cover techniques. To decrease the hazards of floods, safeguard water resources, and promote sustainable development in the Temef Watershed, watershed managers can address the high runoff potential detected using CN values.

To summarize, the CN value is crucial in comprehending the runoff potential and infiltration characteristics of a given location. The CN values suggest a significant likelihood of runoff in the Temef Watershed. The correlation between CN values and infiltration rates underscores the necessity for efficient watershed management measures to regulate runoff and alleviate the hazards linked to flooding. To efficiently manage its water resources and reduce the negative effects of surface runoff, the Temef Watershed can employ strategies to improve infiltration capacity and encourage sustainable landuse practices.

3.1.4. Validation

The CN value for the Temef Watershed is 70.094 for normal conditions, increasing in wet conditions to 84.813, for which the values were obtained from the results of previous research [17], [18]. The analysis showed that the infiltration rate ranged from 1-4 mm/hour due to the conditions of soil lithology and soil texture in the Temef Watershed. The composite CN value from the five sub-watersheds was 69.45 for the AMC II condition.

Validating CN values is essential to guarantee the precision and dependability of the acquired results [19]. This validation step entails the comparison of the calculated CN

values with the observed data or findings from other investigations. By verifying the CN values, researchers may ensure the accuracy and relevance of the results, hence bolstering the credibility of the study's findings.

The fluctuation in infiltration rate can be ascribed to the characteristics of soil lithology and soil texture within the watershed. The infiltration rate is heavily influenced by factors such as the soil's composition, structure, and water retention capacity. Comprehending the correlation between infiltration rate and soil conditions is crucial for evaluating the water-holding capacity and vulnerability to surface runoff of the watershed.

The Antecedent Moisture Condition (AMC) denotes the level of moisture in the soil before to a rainstorm occurrence and has a direct impact on the runoff reaction. The composite CN value is calculated by aggregating the CN values obtained from several sub-watersheds within the Temef Watershed.

This number offers a comprehensive evaluation of the watershed's capacity to generate runoff based on certain moisture conditions. Through the examination of the composite CN value, scientists and managers of watersheds can acquire valuable knowledge about the hydrological patterns of the watershed. This information enables them to make well-informed choices on the management of water resources and the implementation of initiatives to mitigate floods.

To summarize, the process of validating CN values guarantees the precision and dependability of the study's results. The examination of infiltration rates in the Temef Watershed underscores the impact of soil lithology and texture on the process of water absorption. The composite CN value for the AMC II condition provides a full evaluation of the watershed's capacity for runoff [20]. These observations enhance comprehension of the hydrological patterns in the Temef Watershed and reinforce efficient water resource administration and flood mitigation strategies.

4. Conclusion

The curve number value is determined by land cover, soil moisture level, and soil texture. Temef Watershed is dominated by land cover in the form of secondary dry land forests, with a percentage range of 28.50-52.00%. The textural lithology of rocks in the Temef Watershed is dominated by conglomerate and pebbles with a gradation from medium to high; sandy marl, sandstone, tufts, and dacite with a gradation from medium to high; and scaly clay with a gradation from very low to low. The curve number values for each sub- watershed of the Temef Watershed are between 55.10 and 66.12, which are included in the medium gradation category.

It means that the infiltration rate ranges from 1-4 mm/hour due to the lithological and textural conditions of the soil in the Temef Watershed. The composite CN value from the five sub- watersheds is 69.45 for the AMC II condition.

Acknowledgments

The author would like to thank the Nusa Tenggara II River Authority for the data obtained and Prof. Denik Sri Krisnayanti, who has supported and provided much input to the research.

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