3.1. Geographic Properties of Land-Use Change Area
Land cover classification results were the same as the land-use type of the study area. The 1977 and 2016 classification results showed 95% and 98.7% overall accuracy with a Kappa coefficient of 0.94 and 0.97, respectively, indicating a satisfactory level of accuracy. In order to confirm the result of the land-use change from forest to crop field, land-use change map was overlaid on the high-resolution Google Earth imagery and visually interpreted. Orange regions illustrated deforestation (Figure 3
). There is some miss detected region on the stream. However, deforestation was detected well.
Classification results for the 1977 map were as follows: 4.4 km2
(0.3%) of water, 1156.8 km2
(84.6%) of forest, and 205.7 km2
(15.1%) of crop field, while the 2016 map yielded the following results: 5.3 km2
(0.4%) of water, 1098.4 km2
(80.4%) of forest, and 262.5 km2
(19.2%) of crop field. There were many changes in land-use within the study area from 1977 to 2016, particularly a 56.8 km2
(4.9%) change from forest to crop field (Figure 4
The land-use change areas (i.e., forest to crop field) were 25.35 km2 (29.15%), 49.64 km2 (57.09%), 9.30 km2 (10.70%), 2.16 km2 (2.49%), 0.47 km2 (0.54%), and 0.03 km2 (0.03%) in elevation groups of 210 m to 400 m, 400 m to 600 m, 600 m to 800 m, 800 m to 1000 m, 1000 m to 1200 m, and 1200 m to 1450 m, respectively.
The land-use change areas were dispersed by 12.2 km2 (14.0%), 24.9 km2 (28.6%), 22.5 km2 (25.8%), 17.4 km2 (20.1%), and 10.0 km2 (11.5%) at slopes of 0° to 5°, 5° to 10°, 10° to 16°, 16° to 25°, and 25° to 80°, respectively.
The land-use change areas near the main stream and the 2nd, 3rd, 4th, 5th, and 6th streams were 0.6 km2 (0.7%), 8.7 km2 (10.0%), 26.6 km2 (30.7%), 37.7 km2 (43.3%), 11.8 km2 (13.6%), and 1.5 km2 (1.7%), respectively. The maximum change was observed in forest areas near the upper streams with a relatively high elevation (400–600 m) and a steep slope (5–25°).
After the collapse of the Soviet Union in 1989, food aid to North Korea was cut off. In response, the North Korean government changed forest land-use policy, resulting in deforestation of steep slopes to provide land for farms [37
]. According to a report released by the South Korean government in 2004 [7
], 7.9% (972 thousand ha) of North Korea’s total area (12,298 thousand ha) was classified as deforested (i.e., as terraced crop fields).
The reason why the North Korean government changed mountainous forest areas to crop fields has to do with the overall landform of the area. According to the Geomorphon landform classification scheme, the area consists mainly of slopes (509.75 km2, 37.3%), followed by spurs (249.5 km2, 18.3%), hollows (207.6 km2, 15.2%), valleys (153.7 km2, 11.3%), ridges (129.7 km2, 9.5%), flats (63.6 km2, 4.7%), footslopes (28.8 km2, 2.1%), peaks (13.3 km2, 0.9%), shoulders (6.0 km2, 0.4%), and pits (4.8 km2, 0.3%).
Thus, the North Korean government cleared mountainous forest areas for crop fields to solve their food supply problem. The landform of the change area mostly consisted of slopes (4.33 km2
, 44.8%), followed by spurs (1.86 km2
, 19.2%), hollows (1.29 km2
, 13.3%), valleys (0.89 km2
, 9.2%), ridges (0.86 km2
, 8.9%), flats (0.16 km2
, 1.7%), footslopes (0.15 km2
, 1.5%), peaks (0.06 km2
, 0.6%), shoulders (0.05 km2
, 0.5%), and pits (0.02 km2
, 0.3%) (Figure 5
). These results suggest that the areas of land-use change have been transformed into terraced crop fields.
3.2. Spatial Correlation, Soil Loss, and Runoff Change
Network cross K-function analysis was carried out to determine the effects of land-use change at Hoeryeong City on FDAs. As shown in Figure 6
, the curve of observed flooding was above the upper expected curve; therefore, the CSR hypothesis was rejected. Thus, the land-use change was concluded to strongly affect the occurrence of floods. Figure 7
shows the land-use change areas in relation to the FDA locations. Since the topography of the study area was dominated by mountains and most of the river lengths were short and traversed steep slopes, there was a high risk of flood damage [4
]. In addition, the location of terraced crop fields on steep slopes and at high elevations has led to a decline in fertility and productivity [7
]. Inadequate countermeasures against soil loss and post-management strategies have also contributed to devastation in the mountain areas [7
To support the study results, the effects of land-use change on soil loss and runoff change were estimated. Regarding soil loss class, loss of less than 50 tons occurred in 608 km2 (45.1%), 50 to 100 tons in 356 km2 (26.4%), 100 to 150 tons in 198 km2 (14.7%), 150 to 200 tons in 97 km2 (7.2%), and over 200 tons in 88 km2 (6.6%) in 1977. In 2016, the respective affected areas were 576 km2 (42.7%), 359 km2 (26.6%), 202 km2 (15%), 101 km2 (7.5%), and 110 km2 (8.2%). The decrease of less than 50 tons (32 km2, 4.4%) and the increase of more than 200 tons (32 km2, 1.6%) were noticeable.
Regarding land-use change area (i.e., forest to crop field), that losing 50 tons decreased by 25 km2 (29.3%) and that increased by more than 200 tons increased by 14 km2 (16.6%). These results indicate that land-use change brought about the increase in soil loss.
In addition, due to deforestation, the runoff coefficient increased from 0.31 to 0.46. Deforestation involved removing vegetative cover, which made the land cover become bare soil. Bare soil increased the runoff coefficient sharply. It increased surface runoff, which causes erosion of the land surface. As a result, eroded materials were deposited in the stream or river in the study site. It lowered the stream or river depth and overflowed river water flooded in the study site. Rainfall intensity is affected by the time of concentration, which is affected by land cover such as runoff coefficients. If forests are changed to crop fields, the time of concentration decreases, and rainfall intensity increases [35
]. In this study, rainfall intensity increased from 56.3 mm/hr to 60.8 mm/hr because of land-use change. Lastly, peak time runoff increased from 128.2 m3
/sec to 206.6 m3
/sec (Table 5
). Consequently, land-use change from forest to crop field on a steep slope increased soil loss and peak time runoff.
In the summer monsoon season, deforestation results in the vulnerability of the land to flooding and landslides [13
]. As such, steep-slope and forest-to-crop field land-use changes increased soil loss, contributing to flood damage by way of mass wasting, including rockslides and debris flows from terraced crop fields. It was assumed that the rise in riverbed elevation resulting from sediments carried and deposited by heavy rainfall in North Korea monsoon events reduced river and stream drainage capacities [1
In 2016, the study area also experienced levee breaches that contributed to extreme flood damage. As previously mentioned, the North Korean state media described the 2016 flood as the largest natural disaster since 1945 [3
Based on these findings, we concluded that: (1) peak time runoffs and amount of soil loss involving mass wasting (including rockslides and debris flows) have increased in terraced crop fields that were previously forests; (2) debris sediments from upper streams caused an increase in riverbeds and a reduction in drainage capacity; and (3) dike breaches resulted in severe flood damage in the study area. Thus, although the transformation of mountain and hill forests to terraced crop fields in North Korea over the past 30 years has increased agricultural production [4
], these measures have increased the risk of flood disasters and have failed to solve food shortages.
These phenomena have affected change in land-use policy of North Korea. The North Korean government started to consider supporting local communities to restore forest cover on cleared steep slopes [37
]. In 2002, the government initiated the Sloping Land Management Program (SLMP), which aimed to improve ecosystem services on sloping lands by large-scale afforestation of land with a slope greater than 15 degrees [40
]. However, this program was not very successful due to the continuing agricultural activities that rapidly encroached upon sloping lands [37
]. In 2004, the North Korean Ministry of Land and Environment Protection (MoLEP) and the Swiss Agency for Development and Cooperation (SDC) collaborated to implement the SLMP. In 2008, SDC and MoLEP invited the World Agroforestry Centre’s East and Central Asia Office (ICRAF-ECA) to join the SLMP to provide technical advice for land-use planning and agroforestry development [37
]. Consequently, several positive outcomes were observed in the Hwanghae Province, such as an increase in forest cover and establishment of protective grass strips that have helped to reduce soil erosion [37
]. In addition, the North Korean Government developed a National Agroforestry Strategy (NAS) in 2014 to ensure SLMP implementation. The NAS aims to apply agroforestry development to improve people’s livelihoods and food security and to restore upland forest ecosystems by granting user groups the land-use rights to 360,000 ha of sloping land by 2023 [37
The deforestation effects on flood was just investigated at Hoeryeong City. These results can be adapted on the similar geographic properties region in North Korea, especially the east mountainous region. However, deforestation effect on flood at the west of North Korea, where are low elevation and flat regions, should be investigated in the future study. Besides, deforestation was defined forest to crop field in this study; however, deforestation of North Korea has three kinds of patterns such as cultivated (terrace field), unstocked land, and bare mountain. Thus, deforestation should be classified into three categories in the future study.
Despite these limitations in the study, this study is meaningful for the first time to try to investigate the effects of deforestation on the occurrence of floods in North Korea through spatial statistical techniques, changes in soil loss, and the runoff coefficient. Besides, it is meaningful to lay the foundation for evaluating the interrelationship between deforestation and flood in North Korea.