Causes of Land Loss in Tuvalu,a Small Island Nation in the Pacific

Studies on land loss in Tuvalu reveal the following findings. Although both sea level rise and coastal erosion can cause land loss in the tropic Pacific oceanic islands, their mechanisms are different. When sea level rises, the low elevation coastal zone submerges and the erosion datum plane rises, the beach process progresses normally as always, resulting in no beach sediment coarsening. When the sea level is stable, coastal erosion removes finer sediment from reef flat, beach and land, resulting in beach sediment coarsening. The human-induced coastal erosion in the tropic Pacific oceanic islands has the following features. 1) Erosion occurs or intensifies immediately after inappropriate human activities. 2) It occurs near the places having human activities and places related to the above places in sediment supply. 3) It often occurs on original prograding or stable coasts (on lagoon coasts for atolls) because there are more coastal engineering projects and other human activities on such coasts. 4) It is chronic, covering a long period of time. The coastal geological events in Tuvalu islands do not accord with the features resulted from sea level rise but do accord with the features resulted from coastal erosion, particularly from human-induced erosion. The land loss in Tuvalu is mainly caused by inappropriate human activities including coastal engineering and aggregate mining, and partly caused by cyclones. Moreover, all recent measurements (satellite altimetry,thermosteric sea level data and tide observations) so far have not been able to verify any sea level rise around Tuvalu islands.

Assessing Land Loss from Flooding in the Lake St. Martin Basin in Manitoba, Canada

Floodwaters from the Assiniboine River are typically diverted to Lake Manitoba and then Lake St. Martin to save the City of Winnipeg and Portage la Prairie from flooding. The four Indigenous communities living Lake St. Martin basin experienced many negative impacts, including long-term displacement after the 2011 flood. Data analysis of historical water levels of Lake Manitoba and Lake St. Martin were conducted to understand the impact of control structures and water levels. Satellite imagery allowed geographic information system (GIS) raster analysis of the shoreline change of Lake St. Martin before and after the 2011 super flood. From 1986 to 2010, the Lake St. Martin area increased slightly by approximately 0.63% but in 2011 the lake area increased by 13%. 11,000 acres were lost mainly around Lake St. Martin First Nation (LSMFN) reserve and the Little Saskatchewan First Nation, as a result and many houses and other buildings (churches, band offices, etc.) flooded in these communities. The shoreline change analysis showed that, in particular, the west and north shoreline of Lake St. Martin lost its beaches and lakefront properties, to become swampland after the 2011 flood. Thousands of Indigenous people were displaced for eight years, as the community could only start to rebuild its infrastructure and houses on higher ground after a government settlement was reached.

Urban land expansion and arable land loss of the major cities in China in the 1990s

Based on the land-use data in 1990 and 2000, determined by interpreting Landsat Thematic Mapper (TM) imagery, this paper defines the extent of urban construction land, and extracts patches of urban construction land of 145 cities with the largest areas in 1990 and arable land patches around these cities. With these data, this paper analyzes the characteristics of ur- ban construction land expansion and the consequent arable land loss in East, Middle and West China, and further identifies the social, economic and spatial factors of the urban land use changes, using GIS (Geographical Information System) and multivariate regression approaches. The results show that total urban land of the 145 cities expanded by 39.8%, with about 70% of the new urban land converted from arable land in the 1990s. The urban land expansion varied among the three regions, with a value of 43.0% in the East, 33.1% in the West (33.1%) and 17.8% in the Middle. Moreover, mean urban construction land per capita increased by10.7% in the East, but it decreased by 7.7% in the Middle, 1.4% in the West. Statistical analysis indicated that total wages of staff and workers could best explain the differences of urban land expansion.

Rapid Urbanization Induced Extensive Forest Loss to Urban Land in the Guangdong-Hong Kong-Macao Greater Bay Area, China

China has experienced rapid urbanizations with dramatic land cover changes since 1978. Forest loss is one of land cover changes, and it induces various eco-environmental degradation issues. As one of China’s hotspot regions, the Guangdong-Hong KongMacao Greater Bay Area(GBA) has undergone a dramatic urban expansion. To better understand forest dynamics and protect forest ecosystem, revealing the processes, patterns and underlying drivers of forest loss is essential. This study focused on the spatiotemporal evolution and potential driving factors of forest loss in the GBA at regional and city level. The Landsat time-series images from 1987 to2017 were used to derive forest, and landscape metrics and geographic information system(GIS) were applied to implement further spatial analysis. The results showed that: 1) 14.86% of the total urban growth area of the GBA was obtained from the forest loss in1987–2017;meanwhile, the forest loss area of the GBA reached 4040.6 km2, of which 25.60%(1034.42 km2) was converted to urban land;2) the percentages of forest loss to urban land in Dongguan(19.14%), Guangzhou(18.35%) and Shenzhen(15.81%) were higher than those in other cities;3) the forest became increasingly fragmented from 1987–2007, and then the fragmentation decreased from2007 to 2017);4) the landscape responses to forest changes varied with the scale;and 5) some forest loss to urban regions moved from low-elevation and gentle-slope terrains to higher-elevation and steep-slope terrains over time, especially in Shenzhen and Hong Kong.Urbanization and industrialization greatly drove forest loss and fragmentation, and, notably, hillside urban land expansion may have contributed to hillside forest loss. The findings will help policy makers in maintaining the stability of forest ecosystems, and provide some new insights into forest management and conservation.

Nutrient Loss from Various Land-Use Areas in Shixia Small Watershed of Miyun County, Beijing, China

In the Shixia small watershed, twenty experimental plots and two monitoring channels, the residential area and livestock areas were used to monitor the flow, runoff and erosion. Nitrogen and phosphorous concentrations associated with various land uses were analyzed at the same time. The results are presented as follows: (1) TP, TN and COD concentrations of runoff samples in the residential area and livestock areas are nearly 10 times those in other land-use areas. High nutrient loads are associated with village land use, which is due to unsuitable livestock rising. These areas should be treated as the critical areas of non-point source pollution. (2) Different land-use influences intensity the loss of nutrients, especially slope tillings in agricultural land. The amount of nutrient loss from agricultural land per unit is highest, that from forestry is intermediate and that from pastures is lowest. However, in consideration of the variability of land-use areas, agricultural land contributes the greatest to TP and forestry land to TN. (3) The concentrations of TN and TP in sediments from gangues are highest, those in forestry land are intermediate, and those in agricultural land are lowest. Nutrient loss from hilly areas is much greater than from mountainous areas.

IMPACTS OF DIFFERENT TYPES OF LAND USE ON PROCESSES OF SOIL AND WATER LOSS OVER PURPLE SOIL SLOPELAND

Based on natural precipitation observations, impacts of different types of land use on processes of soil and water loss over purple soil related slopeland were studied by simulated rainfall experiments. Measurement data revealed that rainstorms and slope length are the essential factors accountable for soil and water loss on purple soil slopeland for intense rill erosion can be caused on 10 meter long purple soil slopes by high intensity rainfall. Under circumanstances of rainstorms, annual hedge plants grown on slopeland of 25 degrees can cause a reduction of runoff by 22 43 percent and that of erosion induced sand content by 94 98 percent. Stone bund horizontal terraces can lead to a runoff reduction by 62 67 percent in comparison with steep slopelands and that of erosion induced sediment by 97.8 99 percent. Soil and water loss can be substantially decreased on steep slopes by hedge plants with a cost of only 10 20 percent that of the stone bund horizontal terraces. Hence it is an effective way to control soil and water loss in terms of slopeland amelioration and utilization in the Three Gorges Reservoir Area.

Assessment of China's economic loss resulting from the degradation of agricultural land in the end of 20th century

Land degradation is a consequence stemming from both natural processes and social economic activities. On the bases of analyzing general situation of agricultural land degradation in China, the monetary estimating methods such as market value method and shadow engineering method were used to quantitatively assess the economic loss resulting from land deterioration. Results showed that the economic loss in 1999 was 326 81 billion RMB Yuan, which accounted for 4 1% of GDP in the same year of China. If taking five items namely farmland conversion, soil erosion, salinization, decline in reservoir functions, and siltation in waterways and, comparing with that in 1992, the percentage of economic loss to GDP has increased by 1 5 in the only 7 years.

土地经营权入股的隐性失地风险与制度应对

担心失地是制约土地经营权入股的重要因素。基于制度文本和实践纠纷的解析,土地经营权入股的失地风险主要为隐性失地风险——虽然在法律上享有收回土地的权利,但实际上难以收回、有效利用。其主要成因包括入股合同的约定欠周延,农地流转和土地股份组织制度衔接不足,政府投资建设的农业设施的处置规则不明,以及农地纠纷解决机制效能不高。在风险应对方面,不应固守单一的组织法视角,而是需要坚持系统性思维,综合施策予以现实应对,重点包括强化合同治理提高入股合同的周延性,深化农地流转和土地股份组织制度的衔接,健全政府投资建设的农业设施的处置规则,以及构筑更加便捷高效的农地纠纷调处机制。

宽幅条播对燕麦间作箭筈豌豆产量及种间关系的影响

为明确宽幅条播对燕麦‖箭筈豌豆间作系统生产力的影响,在常规条播和宽幅条播两种播种方式下设置燕麦单作、箭筈豌豆单作及燕麦‖箭筈豌豆间作三种种植方式,比较分析了不同处理间作物产量、种间关系及燕麦产量构成因素的差异。结果表明,与单作相比,间作燕麦的饲草及籽粒产量显著提高,增幅分别为24.25%~32.77%和27.44%~32.69%,而间作箭筈豌豆的饲草及籽粒产量降低为7.16%~8.08%和2.80%~5.48%。宽幅条播可提高燕麦及箭筈豌豆产量,宽幅条播间作饲草和籽粒产量的土地当量比较常规条播间作分别提高0.03和0.06。在间作中,燕麦的侵占力大于0,实际产量损失大于1,竞争比率大于1,且燕麦的相对拥挤系数大于箭筈豌豆,说明燕麦在间作系统中处于竞争优势。间作燕麦穗数、穗粒数和穗重较单作显著提高,平均增幅分别为24.23%、14.84%和17.43%,其中宽幅条播间作处理的穗粒数、公顷穗数和穗重均显著高于其他处理。由此可见,宽幅条播下通过与箭筈豌豆间作,可提高燕麦生产力和土地资源利用效率。

快速城市化地区耕地功能性流失测度评价与实证分析

[目的]揭示快速城市化发展下耕地功能性流失现象的阶段性特征,探讨流失类型和流失等级的时空格局与管控策略,为优化区域耕地保护、保障粮食安全提供思路借鉴。[方法]基于快速城市化地区耕地功能性流失的类型特征解析,从“用途—产能—储量”三维视角构建其综合测度体系,监测耕地功能的流失程度并预测流失风险,进一步针对各级流失区域提出管控策略,以江苏省江阴市为研究区开展了实证研究。[结果](1)统筹考虑耕地功能流失的阶段性特征,以此划分耕地功能性流失的主要类型,探索定量表征快速城市化进程下耕地功能性流失程度和风险的方法,对缓解耕地保护与经济发展冲突具有重要理论支撑。(2)江阴市用途流失维中种植非粮食作物面积占比最大,共涉及267个村;产能流失维涉及的村域较少,仅59个村;储量流失中,中低风险类型区涉及的村域较多且数量相当,分别为104个村和115个村。(3)基于综合评价体系,江阴市村域中二级流失占比最大(71.17%),在未来发展中需对不同流失程度实施完善农业建设、优化农业空间布局、加强流失风险监测等差异化治理策略。[结论]耕地功能性流失现象在发生时间、受干扰因素和表现形式等方面存在阶段性特征,其时空格局在分维和综合维度上各具差异,因地施策可为促进区域耕地保护与利用,完善耕地功能风险监测与预警,进而优化国土空间合理布局等提供理论支撑。

不同农作措施下紫色土坡耕地氮磷流失特征

为揭示不同农作措施下径流氮磷流失变化特征,基于2020年12场次的自然降雨产流事件,分析了顺坡无施肥(对照,CK)、顺坡混合施用有机肥和化肥(T1)、顺坡单施化肥(T2)、顺坡单施化肥增量(T3)、横坡单施化肥(T4)5种处理下径流量及径流中总氮(TN)、总磷(TP)质量浓度与流失量、氮磷比(TN/TP)的变化特征.结果表明:①各处理径流TN,TP最高质量浓度均发生在施肥后的前6次降雨,其中的4次降雨事件造成了超50%的总氮、总磷流失.②与单施化肥相比,有机肥化肥配施的径流TN,TP流失量没有显著差异,且径流TN,TP质量浓度与施肥后天数都呈极显著线性负相关(p<0.01).③和顺坡耕作相比,横坡垄作可以显著削减76.85%的径流量、75.12%的径流总氮流失量和87.97%的径流总磷流失量.④CK处理的TN/TP显著高于T2和T3(p<0.05),但与T1,T4差异无统计学意义(p>0.05),所有处理下全年超50%的TN/TP≥50,表现为磷限制.

Spatio-temporal analysis of land-use conversion in the eastern coastal China during 1996-2005

Based on the acquaintance of the regional background of urban-rural transformational development and investigations on the spot,this paper discusses the holistic situation, dominant factors and mechanism of arable land loss and land for construction occupation in the coastal area of China over the last decade,with the aid of GIS technology.Conclusions of the research are summarized as follows：（1）the arable land had been continuously decreasing from 1996 to 2005,with a loss of 1,708,700 hm^2 and an average decrement of 170,900 hm^2 per year;（2）land for construction increased 1,373,700 hm^2 ,with an average increment of 153,200 hm^2 per year;（3）total area of encroachment on arable land for construction between 1996 and 2005 was 1,053,100 hm^2 ,accounting for 34.03%of the arable land loss in the same period,the percentages of which used for industrial land（INL）,transportation land（TRL）,rural construction land（RUL）and town construction land（TOL）are 45.03%,15.8%,15.47%and 11.5%,respectively;and（4）the fluctuation of the increase of construction land and encroachment on arable land in the area were deeply influenced by the nation＇s macroscopic land-use policies and development level of regional economy.The growth of population and advancement of technology promoted the rapid industrialization, construction of transportation infrastructures,rural urbanization and expansion of rural settlements in the eastern coastal area,and therefore were the primary driving forces of land-use conversion.

基于GlobeLand 30的耕地资源损失过程研究——以环渤海地区为例

研究目的:认识和理解耕地资源损失过程。研究方法:基于中国成功研制出的世界上首套30 m分辨率2000—2010年GlobeLand 30数据,利用GIS空间分析方法,综合分析环渤海地区耕地资源损失流向、质量和空间特征。研究结果:2000—2010年,环渤海地区土地利用/覆盖变化剧烈,耕地资源明显减少,主要损失流向为人造覆盖、草地、水体和森林;耕地资源损失区域主要位于降水量较少、海拔较高等耕作条件较差的地区;耕地资源重心总体向东南方向移动,破碎化程度日趋严重。研究结论:基于GlobeLand30数据的耕地资源损失过程研究能有效揭示耕地资源损失流向、质量和空间特征,为区域耕地资源利用与保护提供了积极借鉴和参考。

生物炭施加垄沟集雨种植对水土流失及紫花苜蓿草产量的影响

为缓解我国黄土高原区干旱胁迫和控制水土流失,本研究采用裂区设计,生物炭施加模式(生物炭施加和无生物炭施加)为主区,耕作模式(打结垄、开敞垄和平作)为副区,研究不同生物炭施加模式和耕作模式对土壤水分、径流、泥沙流失、养分流失、紫花苜蓿(Medicago sativa L.)干草产量和水分利用效率的影响。试验结果表明,在2019和2020年紫花苜蓿生育期,与平作处理相比,开敞垄和打结垄处理的径流量、泥沙、全氮、全磷、有机质流失量分别减少20.1%~37.7%和60.1%~64.7%,54%~75.5%和77.1%~87.6%,44.4%~65.5%和49.0%~81.8%,36.7%~75.3%和48.6%~87.3%,48.2%~72.9%和58.6%~85.4%,土壤贮水量、紫花苜蓿干草产量和水分利用效率分别增加39.5~52.1和31.2~60.5 mm,26.2%~31.7%和26.5%~35.2%,10.07~14.86和12.14~16.55 kg·hm^(-2)·mm^(-1)。与无生物炭施加处理相比,生物炭施加处理的径流量、泥沙、全氮、全磷和有机质流失量分别减少48%~69%,94%~125%,12%~24%,28%~46%和35%~48%,土壤贮水量、紫花苜蓿干草产量和水分利用效率分别增加42.93~26.09 mm,106%~111%和1.96~6.90 kg·hm^(-2)·mm^(-1)。本研究表明,生物炭施加打结垄沟集雨种植是该区域适宜耕作模式。

坡位与利用方式对红壤磷素储存容量的影响差异——以江西鹰潭孙家小流域为例

为明确坡位与利用方式对土壤磷素储存容量(SPSC)的影响差异,以更加准确地评估坡耕地红壤磷的流失风险,本研究以江西鹰潭孙家小流域内不同坡位(坡顶、坡上、坡中、坡底)及不同利用方式(稻田、花生旱地、橘园)的表层土壤为研究对象,对比分析了SPSC的变化规律与差异,探讨了土壤全碳、全氮、全磷、有效磷、铁铝氧化物含量等因子对SPSC的影响。结果表明:不同坡位的稻田、花生旱地与橘园的SPSC变化范围分别为-410.9~-137.8、-283.8~-128.0 mg·kg^(-1)及-280.6~-36.3 mg·kg^(-1);且随坡位的降低,稻田(坡底除外)与橘园SPSC显著降低,而花生旱地SPSC则增加。与稻田土壤相比,同一坡位花生旱地与橘园SPSC相对增加;相关分析表明,不同坡位的稻田、花生旱地及橘园SPSC与土壤非晶质铁铝氧化物、游离态铁铝氧化物极显著正相关,且游离态氧化铝的影响相对最大,而与有效磷、总碳、总氮则显著负相关。江西鹰潭孙家小流域内红壤坡耕地SPSC均为负值,意味着此时的土壤均为磷源,土壤磷均具有流失风险;坡底的稻田及橘园、坡顶花生旱地土壤磷的流失风险相对较高,应及时采取相应的调控措施以有效降低土壤磷素的流失潜能与风险。

基于RUSLE模型的秦岭—大巴山地土壤侵蚀时空特征分析

[目的]秦岭—大巴山地(秦巴山地)是我国重要的南北地理—生态过渡带主体,对秦巴山地的土壤侵蚀研究将有助于该区域的生态保护和水土资源管理。[方法]基于RUSLE模型计算秦巴山地的土壤侵蚀模数,并量化分析了该区域的土壤侵蚀的时空分布格局。[结果](1)2000—2020年秦巴山地的微度侵蚀面积呈上升趋势,轻度侵蚀及其以上等级的土壤侵蚀面积均呈下降趋势;从空间来看,秦巴山地东北和西南部的土壤侵蚀等级较高,中间较低;(2)秦巴山地的土壤侵蚀相对集中在500~1500 m、坡度15°~25°区域内;(3)秦巴山地发生土壤侵蚀最主要的土地利用类型为林地,耕地、林地的微度侵蚀以及草地的微度、剧烈侵蚀面积呈上升趋势;(4)秦巴山地土壤侵蚀主要分布在陕西、四川和甘肃,且甘肃和四川的剧烈侵蚀呈上升趋势。[结论]2000—2020年秦巴山地的侵蚀面积和强度呈“双下降”的态势,其整体侵蚀状况好转,但侵蚀分布存在明显空间差异。

基于多任务学习的近岸舰船检测方法

在遥感光学图像近岸舰船目标检测任务中,针对近岸复杂场景中存在形状近似目标的虚警问题,提出一种基于多任务学习的近岸舰船目标检测方法。该方法通过构建海陆分割任务与舰船检测任务并行双路框架,将传统的任务串行处理流程优化为并行处理方式,设计联合损失函数进行双路优化训练约束,提升模型训练的稳定性,通过双分支融合模块剔除陆地掩膜中的检测结果,实现陆地虚警滤除。采用谷歌地球遥感图像制作的数据集进行实验,将本文提出的方法与单任务检测算法YOLOv5相比,mAP提升了4.4个百分点,虚警率降低了3.4个百分点。实验结果表明本文算法对陆地虚警抑制有效。

基于自然-经济综合视角的碳排放强度与生态盈亏多情景模拟研究——以淮海经济区为例

生态盈亏与碳排放强度是衡量生态环境质量与经济发展水平的重要指标,分析两者时空关联并预测未来演进方向对提升区域生态环境质量健康水平具有指导作用。以淮海经济区为研究对象,以2000年、2010年和2020年3期土地利用数据为基础,运用PLUS模型、双变量自相关、耦合协调度及面板计量回归模型等方法,预测并分析淮海经济区2000—2036年3种情景下碳排放强度、生态盈亏的演化特征及时空关联。结果表明:(1)常规发展情景下,淮海经济区生态性呈下降趋势,强耕地保护情景下生态性微有下降,强生态保护情景下,生态性有所上升,自然要素是主导。(2)常规发展情景下,淮海经济区碳排放量逐渐加大,强耕地保护及强生态保护情景下,则呈下降趋势;三种状态下,碳排放强度均呈下降趋势,时间轴2010—2020年降幅较大,空间序河流水系地区碳排放强度降幅较大。(3)淮海经济区碳排放强度与生态盈亏呈负相关关系,低低和低高聚类主要分布在城镇建设用地周边,高低和高高聚类分布在人类干扰较少的自然区域;高高聚类主要位于河流水系交汇区,表明水域有效降低碳排放强度的功能。(4)淮海经济区的碳排放强度与生态盈亏之间的耦合协调度呈东西高、中部低的态势,高耦合协调度主要分布于低高聚类,即自然地区域,低耦合协调度主要分布于西部平原区;科学技术、经济发展水平及二产占比对耦合协调度呈正影响,城镇化率、道路密度呈负影响。研究认为,淮海经济区当前的土地利用方式,不利于区域环境质量的改善,建议进一步强化建设用地管控、大力推进高标准农田建设和水域保护,以耕地质量提升和水域保护加强提升粮食主产区环境质量健康水平。

不同有机物料对滩涂盐碱地有机碳损失·重金属含量及水稻产量的影响

通过田间小区试验,研究不同有机物料(对照CK、鸡粪T_(1)、羊粪T_(2)、菌菇渣T_(3)、生物腐植酸T_(4))在等有机碳投入量下滩涂盐碱地中有机碳损失、土壤各类有机质分布及其对土壤重金属含量及水稻产量的影响。结果表明,不同有机物料施入土壤后,T_(1)、T_(2)、T_(3)、T_(4)有机碳损失率分别为64.52%、58.69%、47.04%和29.56%;未损失部分转化为土壤有机质,虽然T_(3)处理有机碳损失率较低,但土壤中的活性有机质占比低于其他处理。T_(3)、T_(4)能降低滩涂盐碱地土壤中的有效态汞、砷、镉、铅、铬含量,其中以T_(4)效果最好,T_(1)、T_(2)处理能降低汞、镉、铬有效态含量,但在一定程度上分别增加有效态砷、有效态铅含量。T_(1)~T_(4)对水稻有效穗、穗粒数有显著提高作用,较对照分别增产10.17%、14.03%、6.65%和24.52%,其中T_(4)能显著增加水稻穗粒数。由此可见,在滩涂盐碱地综合改良利用中,生物腐植酸在有机碳损失率、增加土壤活性有机质、降低土壤重金属含量及提高水稻产量方面具有较显著优势,是滩涂盐碱地改良的优良有机物料投入品。

Study on Cultivated Land Resource Situation and Its Macro-Driving Forces in Yantai Area

According to the statistics of cultivated land in Yantai area during the Reform and Opening-up over the past 20 years and with the detailed survey and changing data of land use during the recent live years, this paper reveals the basic characteristics of the resource utilization, the basic process of the changes, regional differences, and current direction of cultivated land In addition, this paper makes a preliminary research on the major driving factors and the influence mechanism for the changes in the cultivated land area. and reaches the following conclasions： （1） There was a trend of obvious fluctuant decrease in cultivated laud area during the past 20 years. The changes of cultivated land have undergone the process - slow decrease,steep decrease, slow decrease. And there were three peak periods of cultivated land loss around 1985. 1987-1996 and 1991-1993 respectively （2） The decreased cultivated land was mainly converted into land for industry and mining, residential area. and all kinds of traffic and orchards. The increase of cultivated land mainly came from exploitation and land and arrangement and reclamation of industry and mining land （3） The economic development. population growth and policy are the dominant macro-driving factors for the decrease of cultivated land area in Yantai. The obvious sudden changes during the change process of cultivated land acreage are relevant to the national macro.policies background The rapid decrease of cnltivated land area is synchronous with the overheated economic growth resulting from the direct investment in fixed assets. Spatial distribution of the decrease in cultivated land is congruous with the differences in speed and the scale of economic growth between the different regions of Yantai area. Furthermore. the decrease in cultivated land caused by population increase cannot be ignored.