Pengaruh Tutupan Lahan terhadap Base Flow Index DAS Way Seputih Provinsi Lampung

Tujuan penelitian ini adalah menganalisis nilai Base Flow Index (BFI) dan debit baseflow DAS Way Seputih; menganalisis nilai Curve Number (CN) composite DAS Way Seputih tahun 2005, 2009, dan 2011; serta menganalisis pengaruh perubahan tutupan lahan terhadap baseflow DAS Way Seputih. Penentuan baseflow tahun 1973-2006 menggunakan Metode RDF (Recursive Digital Filter) terbaik berdasarkan uji kinerja, yaitu Root Mean Square Error (RMSE), R2, dan grafik FDC (Flow Duration Curv) di antara metode Lyne&Hollick, EWMA, Chapman Algorithm, dan Nathan and McMahon. Kemudian Metode RDF terbaik ini dibandingkan dengan Model SCS-CN (Soil Conservation Sevice Curv Number) untuk menentukan baseflow dan BFI tahun 2005, 2009, dan 2011 berdasarkan uji kinerja. BFI DAS Way Seputih tahun 2005, 2009, dan 2011 berturut-turut adalah 0,8178; 0,54225; dan 0,649502. Untuk nilai CN I composite, CN II composite, dan CN III composite tahun 2005 berturut-turut adalah 63,9191; 79,8869; dan 80,1315. Nilai CN I composite, CN II composite, dan CN III composite tahun 2009 berturut-turut adalah 42,7834; 60,8427; dan 83,6676. Sedangkan Nilai CN I composite, CN II composite, dan CN III composite tahun 2011 berturut-turut adalah 66,4066; 81,9460; dan 94,5943. Nilai BFI dipengaruhi oleh debit baseflow dan debit sungai. Debit sungai dipengaruhi oleh curah hujan, sedangkan debit baseflow dipengaruhi oleh perubahan luas tiap jenis tutupan lahan.

Correction: Kelly, L., et al. Quantification of Temporal Variations in Base Flow Index Using Sporadic River Data: Application to the Bua Catchment, Malawi. Water 2019, 11, 901

In the published article [...]

Base Flow Index of Way Seputih River at Buyut Udik

One of river flow components considered in watershed management is baseflow, the main contributor to the river flow during dry season. Ratio between baseflow to total flow is called Base Flow Index (BFI). In fact, BFI can be used as an indcator how good the catchment hydrology is. The purpose of this study is to analyze BFI of Way Seputih river at Hydrometry Station Buyut Udik. The method includes Recursive Digital Filter (RDF) method, which utilizes constant recession of hydrographs in displaying the ratio of baseflow from continuous discharge during periods when there is no direct runoff. Daily discharge data from Way Seputih river at Buyut Udik recorded from 1973 to 2006 is used. The result shows that yearly BFI values range from 0.142 to 0.487 and there is a decrease trend of yearly BFI over time. This result is confirmed by yearly average discharges which tend to decrease over time. These are effected by land cover degradation in Way Seputih catchment and this should be a concern for authorities to improve land and water conservation in Way Seputih catchment so that river flow could be maintained throughout the year. Keywords: base flow index, baseflow, dry season

Uji Validasi Data Debit

Uji validasi data merupakan langkah pemeriksaan untuk memastikan data hidrologi yang disajikan telah sesuai dengan kriteria yang ditetapkan, merupakan salah satu bagian dari proses kendali mutu. Teknik validasi dirancang untuk mendeteksi kesalahan umum yang mungkin terjadi dan hasil validasi didesain untuk menunjukkan alasan mengapa suatu nilai data ditandai. Tujuannya adalah menganalisis data dengan metode uji validasi homogenitas dan trend, mendeteksi penyimpangan data, dan memberikan informasi kondisi kualitas data dengan jenis kategori. Uji validasi data debit dengan studi kasus DAS Citarum Hulu dilakukan pada 4 pos duga air terpilih menggunakan metode Pettitt dan T untuk uji homogenitas, metode Mann-Kendall dan Spearman untuk uji trend pada tingkat signifikansi 5%. Hasil kajian menunjukkan bahwa uji homogenitas dengan metode Pettitt lebih baik diterapkan pada kajian ini karena memiliki populasi data debit yang tidak berdistribusi normal. Kelebihan dari metode Pettitt adalah dapat menguji data tanpa adanya asumsi harus berdistribusi normal. Hasil uji trend dengan metode Mann-Kendall dan Spearman menunjukkan hasil signifikansi yang relatif sama, karena kedua uji tersebut memiliki metode statistik non parametrik. Kekuatan kedua uji ini tergantung pada tingkat signifikansi, ukuran sampel data, dan jenis distribusi. Hasil uji menunjukkan kondisi data di pos Citarum-Nanjung, Cigulung-Maribaya, Cikapundung-Maribaya dan Cikapundung-Gandok, masing-masing berkategori Baik, Tidak Realistis, Meragukan dan Meragukan. Homogenitas dan trend data debit di DAS Citarum Hulu tidak merata. Trend data debit di sub DAS Cikapundung mengalami kenaikan secara signifikan mulai tahun 1980an dan mengalami penurunan trend BFI di pos Cikapundung-Maribaya pada tahun 1990-2013. Hal ini mengindikasikan adanya pengaruh perubahan fungsi lahan pada sub DAS Cikapundung.Kata Kunci: Validasi data, data debi, Mann Kendall, Pettitt, Base flow Index (BFI)

Assessing the characteristics and drivers of compound flooding events around the UK coast

In low-lying coastal regions, flooding arises from oceanographic (storm surges plus tides and/or waves), fluvial (increased river discharge), and/or pluvial (direct surface run-off) sources. The adverse consequences of a flood can be disproportionately large when these different sources occur concurrently or in close succession, a phenomenon that is known as “compound flooding”. In this paper, we assess the potential for compound flooding arising from the joint occurrence of high storm surge and high river discharge around the coast of the UK. We hypothesise that there will be spatial variation in compound flood frequency, with some coastal regions experiencing a greater dependency between the two flooding sources than others. We map the dependence between high skew surges and high river discharge, considering 326 river stations linked to 33 tide gauge sites. We find that the joint occurrence of high skew surges and high river discharge occurs more frequently during the study period (15–50 years) at sites on the south-western and western coasts of the UK (between three and six joint events per decade) compared to sites along the eastern coast (between zero and one joint events per decade). Second, we investigate the meteorological conditions that drive compound and non-compound events across the UK. We show, for the first time, that spatial variability in the dependence and number of joint occurrences of high skew surges and high river discharge is driven by meteorological differences in storm characteristics. On the western coast of the UK, the storms that generate high skew surges and high river discharge are typically similar in characteristics and track across the UK on comparable pathways. In contrast, on the eastern coast, the storms that typically generate high skew surges are mostly distinct from the types of storms that tend to generate high river discharge. Third, we briefly examine how the phase and strength of dependence between high skew surge and high river discharge is influenced by the characteristics (i.e. flashiness, size, and elevation gradient) of the corresponding river catchments. We find that high skew surges tend to occur more frequently with high river discharge at catchments with a lower base flow index, smaller catchment area, and steeper elevation gradient. In catchments with a high base flow index, large catchment area, and shallow elevation gradient, the peak river flow tends to occur several days after the high skew surge. The previous lack of consideration of compound flooding means that flood risk has likely been underestimated around UK coasts, particularly along the south-western and western coasts. It is crucial that this be addressed in future assessments of flood risk and flood management approaches.

Quantification of Temporal Variations in Base Flow Index Using Sporadic River Data: Application to the Bua Catchment, Malawi

This study investigated how sporadic river datasets could be used to quantify temporal variations in the base flow index (BFI). The BFI represents the baseflow component of river flow which is often used as a proxy indicator for groundwater discharge to a river. The Bua catchment in Malawi was used as a case study, whereby the smoothed minima method was applied to river flow data from six gauges (ranging from 1953 to 2009) and the Mann-Kendall (MK) statistical test was used to identify trends in BFI. The results showed that baseflow plays an important role within the catchment. Average annual BFIs > 0.74 were found for gauges in the lower reaches of the catchment, in contrast to lower BFIs < 0.54 which were found for gauges in the higher reaches. Minimal difference between annual and wet season BFI was observed, however dry season BFI was >0.94 across all gauges indicating the importance of baseflow in maintaining any dry season flows. Long term trends were identified in the annual and wet season BFI, but no evidence of a trend was found in the dry season BFI. Sustainable management of the investigated catchment should, therefore, account for the temporal variations in baseflow, with special regard to water resources allocation within the region and consideration in future scheme appraisals aimed at developing water resources. Further, this demonstration of how to work with sporadic river data to investigate baseflow serves as an important example for other catchments faced with similar challenges.

Assessing the characteristics and drivers of compound flooding events around the UK coast

In low-lying coastal regions, flooding arises from oceanographic (storm surges plus tides and/or waves), fluvial (increased river discharge) and/or pluvial (direct surface runoff) sources. The adverse consequences of a flood can be disproportionately large when these different sources occur concurrently, or in close succession, a phenomenon that is known as ‘compound flooding’. In this paper, we assess the potential for compound flooding arising from the joint occurrence of high storm surge and high river discharge around the coast of UK, using observed sea level and river discharge data. First, we map the spatial dependence between high skew surges and high river discharge, considering 326 river stations linked to 33 tide gauge sites. We find that the joint occurrence of high skew surges and high river discharge occurs more frequently during the study period (15–50 years) at sites on the south-west and west coasts of the UK (between 3 and 6 joint events per decade), compared to sites along the east coast (between 0 and 1 joint events per decade). Second, we investigate the meteorological conditions that drive compound (i.e. joint occurrence of high skew surge and high river discharge) and non-compound events (i.e. high skew surge or high river discharge only) events across the UK. We show, for the first time, that spatial variability in the dependence and number of joint occurrences of high skew surges and high river discharge is driven by meteorological differences in storm characteristics. On the west coast of the UK, the storms that generate high skew surges and high river discharge are typically similar in characteristics and track across the UK on comparable pathways. In contrast, on the east coast, the storms that typically generate high skew surges are mostly distinct from the types of storms that tend to generate high river discharge. Third, we briefly examine how the phase and strength of dependence between high skew surge and high river discharge is influenced by the characteristics (i.e. flashiness, size, elevation gradient) of the corresponding river catchments. We find that high skew surges tend to occur more frequently with high river discharge at catchments with a lower base flow index, smaller catchment area and steeper elevation gradient. In catchments with a high base flow index, large catchment area and shallow elevation gradient the peak river flow tends to occur several days after the high skew surge. The previous lack of consideration of compound flooding means that flood risk has likely been underestimated around UK coasts, particularly along the southwest and west coasts. It is crucial that this is addressed in future assessments of flood risk and flood management approaches.

Controls on hydrologic drought duration in near-natural streamflow in Europe and the USA

Climate classification systems, such as Köppen–Geiger and the aridity index, are used in large-scale drought studies to stratify regions with similar hydro-climatic drought properties. What is currently lacking is a large-scale evaluation of the relation between climate and observed streamflow drought characteristics. In this study we explored how suitable common climate classifications are for differentiating catchments according to their characteristic hydrologic drought duration and whether drought durations within the same climate classes are comparable between different regions. This study uses a dataset of 808 near-natural streamflow records from Europe and the USA to answer these questions. First, we grouped drought duration distributions of each record over different classes of four climate classification systems and five individual climate and catchment controls. Then, we compared these drought duration distributions of all classes within each climate classification system or classification based on individual controls. Results showed that climate classification systems that include absolute precipitation in their classification scheme (e.g., the aridity index) are most suitable for differentiating catchments according to drought duration. However, differences in duration distributions were found for the same climate classes in Europe and the USA. These differences are likely caused by differences in precipitation, in catchment controls as expressed by the base flow index and in differences in climate beyond the total water balance (e.g., seasonality in precipitation), which have been shown to exert a control on drought duration as well. Climate classification systems that include an absolute precipitation control can be tailored to drought monitoring and early warning systems for Europe and the USA to define regions with different sensitivities to hydrologic droughts, which, for example, have been found to be higher in catchments with a low aridity index. However, stratification of catchments according to these climate classification systems is likely to be complemented with information of other climate classification systems (Köppen–Geiger) and individual climate and catchment controls (precipitation and the base flow index), especially in a comparative study between Europe and the USA.