Assessment of Land Resources Potentials for Orientation of Agriculture Land Use in Ham Thuan Bac district, Binh Thuan province

Land resources potential assessment is a valuable method for orienting land use planning in the world as well as in Vietnam. This study aimed to (1) assess land suitability and (2) orient the effective use of agricultural land in the study area. Results indicated that Ham Thuan Bac had 107 land mapping units (LMUs) with 11 main landuse types (LUTs) such as rice, cassava, vegetables, dragon fruit, and other trees. Agricultural land with high (S1) and moderate suitability (S2) for most LUTs was small; however, marginal (S3) and not suitability (N) lands were quite large. Results of land suitability evaluation on current land use showed that land area of annual crops was mainly found at the S3 suitability level, specifically for the rice area, 67.75% was marginally suitable and 23.29% of the area was unsuitable; In terms of perennial trees, except for dragon fruits which were mainly cultivated at the S2 suitability level (78.28%), the others such as cashew and other fruit trees were S3 and N levels. Based on land suitability classification, for mainstay LUTs, we proposed to keep 8550.0ha with S1, S2, and a part of S3 area to be irrigated for cultivating rice; 9071.7ha of dragon fruit land including 694.67ha of S1 and 8377.03ha of S2 land; only cultivate annual crops, fruit trees and grazing grass on the land with suitable levels from S1 to S3 of the current status; and the remaining areas of S3 and N levels for rice and areas of S2, S3, or N levels for dragon fruit would be converted to lands for annual crops, fruit trees, or grass for livestock.

the process of land use planning (LUP) when land resources are limited (Bandyopadhyay et al., 2009). The rapid population growth, as well as human activities, has created pressure on agricultural land. Intensified farming activities without proper land management practices lead to the degradation of land (Abdelrahman et al., 2016). Land evaluation approach allows for identifying fundamental limiting factors for agricultural production, provides information in terms of constraints and opportunities for the utilization of land, and then guides decisions based on the optimal prerequisite for land use planning and development (Motuma et al., 2016;Truong et al., 2014;Pham Quang Vinh & Pham Thi Thanh Huong, 2012). Thus, land evaluation is implemented before recognizing the need for some changes in land use such as agricultural development or forestry plantation schemes.
Land suitability assessment plays an important role in agricultural development and future planning (Belal et al., 2014). This has been implemented to help decision-makers and agriculture development planners determine the optimal use of the land. Assessing land suitability for various crops is necessary to choose the right crop and variety for the area based on matching the suitability of the area with the crop. Various methods of land evaluation have been developed throughout the years (FAO, 1976;Sys, 1985). The Food and Agricultural Organization (FAO, 1976) recommended an approach for land suitability evaluation for crops in terms of suitability ratings ranging from highly suitable to not suitable based on climatic and terrain data and soil properties. Sys (1985) proposed a range of capability indexes to denote soil limitations for crop production. A land suitability assessment could be viewed from three different perspectives: (i) productivity; (ii) workability; and (iii) sustained use of the land (Baja et al., 2001). With advances in information and communication technology, land evaluation through map analysis techniques based on the theoretical framework from FAO has been completed using a geographical information system (GIS) to analyze land capability to sustain a defined land use.
Ham Thuan Bac, a mountainous district of Binh Thuan province, has an agricultural land area of approximately 124 thousand hectares. Land resources are playing an important role in the economic development of the district. However, a serious constraint in this district is the lack of irrigation water during the dry season. Drought is one of the major factors limiting crop production in the area. Therefore, to maximize the effectiveness of the land resources, it is necessary to assess land suitability for agricultural production in Ham Thuan Bac district (Ngo Thanh Son et al., 2018;Hoang Le Huong et al., 2018). In drought-affected conditions, irrigation restricted areas are not suitable for crops that require water such as rice. Hence, it is an important step for the orientation of agricultural land use planning in the future.
The objectives of the study were: (1) to assess land suitability; and (2) to orient an effective use of agricultural land in Ham Thuan Bac district. The results of the study could help to estimate the potential conversion of agricultural land and to recommend an effective agricultural land use system.

Study Area
Ham Thuan Bac district is located in Binh Thuan province, Southern Central Coast of Vietnam (Figure 1). The natural area is 1347.21km 2 (Figure 1) with a population of 173,253 people (2017). In this area, the topography is quite diversified including semimountainous terrain, alluvial plain, and coastal sand. The climate, characterized by the tropical monsoon region, is divided into two seasons, namely the rainy season from May to October and the dry season from November to April. The average temperature is approximately 26 o C to 28 o C and the annual total precipitation varies from 1070mm (delta areas) to 1995mm (high mountains). However, precipitation is highly variable and unevenly distributed. Due to reduced rainfall in the dry season (11% of the total annual rainfall), the dry season often causes serious water shortages for food production and people's daily needs. According to reports from the Department of Agriculture and Rural Development at Ham Thuan Bac district (2017), agricultural activities in many areas still depend on rain.

Methodology
The assessment of land quality for a specific type of land use is based on land use requirements and constraints. Such requirements and constraints are then used as the basis for establishing what is termed 'evaluation criteria' (Figure 2).
Regarding these 'evaluation criteria', a set of algorithms is then employed to match the existing quality of land and the requirements of that particular type of land use.
The matching procedure (FAO, 1976 and after) then establishes a ranking of the potentials of land for a given purpose. The final result of the land evaluation is a map that portrays the divisions of the area of interest into suitability classes or land unit indices for nominated land use. The first is produced from an approach called a categorical system and the second from a continuous method of land suitability evaluation.
There are at least two important groups of land attributes for land suitability analysis: inherent qualities of soils and external characteristics. In this study, soil attributes, topographic elements, and management were used to represent all groups.
Land-use factors are land use characteristics in the study area such as land-use type or land cover, thereby helping to define the land attributes requirements appropriately. This study focuses on agricultural land uses (annual and perennial crops), identified through data collection and field surveys.
The analytical procedure developed consisted of the following steps (

Step 1. Determination of individual ratings of land characteristics
Each of the land attributes within variables in each group (i.e., soils, topography, and management) was first independently examined and rated from the collected data. The rating of soil attributes was done based on the available soil map which had 'crisp' boundaries, irrigation areas identified from water resources data, while that of topography was based on the DEM which had continuous values.
The irrigation area was determined based on the irrigation map of Binh Thuan province in combination with the collected primary data. There were 88,676.01ha (65.95% of agricultural and forest land) which depended on rain (or irrigation regime from rainwater), about 23,998.99ha (17.85%) that was actively irrigated, and the remaining 12,595.81ha (9.37%) that was a semi-active irrigation regime.

Step 2. Group ratings of land characteristics
Land attributes within each group were then combined using a 'combination' function. The result was land capability or land mapping units indicating areas with qualities that differed sufficiently from other land units to affect their suitability for different land uses (FAO). Land Mapping Units (LMUs) of the Ham Thuan Bac district were built in the area of agricultural land and unused land by overlaying thematic maps including soil types, slope, thickness, soil texture, and irrigation regime (Figure 4). These were the factors that had the greatest influence on farming of the study area, and in particular, there were regional differences in the study area, thus it determined the difference in the suitability of land use.

Step 3: Calculation of overall land suitability indices
Land-use requirements are described by the land qualities needed for sustainable production; land quality is a complex rating of land that has a direct effect on its use (evaluation criteria).
At this stage, based on the cell-by-cell operation in the GIS database, all groups of land variables and land requirements were utilized to produce the overall indices of land suitability for cropping. The present study used the four-level land suitability classification commonly used by FAO (1976FAO ( , 1983FAO ( , 1985FAO ( , and 2007 including: S1 -Highly suitable, Land having no significant limitations to the sustained application of a given use or only minor limitations that will not significantly reduce productivity or benefits and will not raise inputs above an acceptable level; S2 -Moderately suitable, Land having limitations which in the aggregate are moderately severe for sustained application of a given use; the limitations will reduce productivity or benefits and increase required inputs to the extent that the overall advantage to be gained from the use, although still attractive, will be appreciably inferior to that expected on Class S1 land; S3 -Marginally suitable, Land having limitations which in the aggregate are severe for sustained application of a given user and will so reduce productivity or benefits, or increase required inputs, that this expenditure will be only marginally justified; and N -Not suitable, Land which has qualities that appear to preclude sustained use of the kind under consideration.

Step 4: Interim matching of land-use requirements with actual land qualities
Interim matching of land-use requirements with actual land qualities: matching (i.e., suitability assessment) for each land-mapping unit could be considered only for the physiological requirements of a specific crop(s) and the existing biophysical land conditions. The first stage in matching was to compare the requirements of each land-use type with the land qualities of each land unit. The simplest procedure was to check the measured values of each land quality or characteristic against the class limits, and then allocate each land unit to its land suitability class according to the most severe limitation.
For cases in which at least one limitation was enough to render the land unsuitable for use, the method of taking the most severe limitation was valid. For example, for maize cultivation, it was of no use having level land and sufficient rainfall if the soils were highly saline. For less severe values of limitations, alternative methods of combining ratings for individual qualities could be used.
Step 5: Final matching The interim suitability classifications produced in the preceding step might be reevaluated taking into consideration a range of additional factors, e.g., potential land improvements, environmental impacts, and economic and social analysis. This is a key step to help managers/planners make the most appropriate decision.

Land use requirement
Based on the current land use map (Department of Natural Resources and Environment of Ham Thuan Bac district, 2017) and the results of the survey on crop distributions in the district, there were 11 LUTs identified for agricultural purpose in Ham Thuan Bac district (Table 4 and Figure 5).
As can be seen in Table 4, the fruit tree area occupied the largest area (31,348.82ha), of which dragon fruit accounted for 61.73% of orchard land (or 32% of total agricultural land -19,352.71ha), whereas other fruit trees including custard-apple, avocado, jackfruit, banana, mangosteen, and mango occupied 11,996.11ha (38.27%). The annual crop was predominantly paddy rice (12,588.97ha); however, rice cultivation was dependent on rain and irrigation conditions. Thus, paddy rice could be transplanted from 2 to 3 seasons a year. Other annual crops such as peanuts, beans (5,696.45ha), cassava and sweet potato (3,070.56ha), and maize (2,372.07ha) were widely cultivated in Ham Thuan Bac. Rice was mainly cultivated in drought-prone areas, therefore, in the winter-spring season, rice and other crops were often strongly affected by drought due to water shortage. In contrast to other LUTs, grass for livestock was not allocated in the district's land use planning. Local people often grew grass in the land of annual crops or gardens, so the land area of this LUT was negligible (3.7ha).
In this paper, the suitability criteria (land characteristics) for land use types were divided into four levels from highly suitable to not suitable ( Table 2).
Limiting values are the values of a land quality or land characteristic that determine the class limits of land suitability for a certain use. In orientation and land-use planning, the first and most important decision is to determine whether or not the land is suitable or unsuitable for production.

Land suitability classification
Based on the land characteristics and land use requirements of the LUTs of Ham Thuan Bac district, the land suitability classification was determined for 11 land-use types for 61,126.09ha of agricultural production land in Ham Thuan Bac district (Table 3). Table 3 demonstrates the maximum suitable total area (S1/S2/S3) for each type of land use that could be obtained under the current conditions of the study area. The results of land suitability classification for each LUT separately indicated that for LUTs including rice, cashew, rubber, coffee, and other fruit trees (1, 8, 9, 10, and 11), if grown on the entire existing area, more than 50% of the area would be unsuitable. The remaining LUTs would be unsuitable lands, if cultivated in the entire existing area, which would vary from 6,579.72ha to 15,377.54ha, (10.76% to 25.16%) of the agricultural land. The suitability assessment for LUTs in the Ham Thuan Bac district showed that there was a small area that was highly suitable (S1) for all crops. In terms of moderate suitability level, most of LUTs had a certain area of compliance with S2 suitability level, the largest areas were cultivating dragon fruit (46.24%) and grass (54.87%). Under the marginal suitability level, if all LUTs were cultivated on the entire area, had a quite large part of the area only reaching this level, ranging from 19,265.92ha (32.52%) to 44,740.94ha (73.19%). The main limiting factors for all LUTs were inadequate rainfall and irrigation.

The orientation of agricultural land use
According to survey results and statistics (Department of Agriculture and Rural Development of Ham Thuan Bac district, 2017; 2018), the actual cultivated areas of LUTs were much different from LUTs under the land use plan in 2017 (Table 5).
Currently, the area of rice cultivated was from 8,515 to 9,232ha depending on the season, whereas the cultivated rice area under the landuse plan in the district was 12,588.97ha, land-use efficiency was about 73%. The same was observed in all LUTs in the district. The reason for its difference is that Not suitability (N) or Marginal suitability (S3) occupied a majority of   Thus, based on the actual land use in Ham Thuan Bac district, we proposed to keep stable rice cultivation in the area of 8550ha (including 1,127.68ha of rice under S1 and S2 suitability levels and 7,422.32ha rice under S3 suitability level) due to light soil texture (S3) but with active irrigation regime. The remaining area should be converted to other annual crops including rice area with remaining S3 and N suitability levels (due to yellowish-red soil). More importantly, due to the consumption market, the actual cultivated area of dragon fruit was only 9,000ha, which was much lower than the approved land use plan (19,352.71ha) of the district. Therefore, for this LUT, we recommend cultivating dragon fruit under the S1 suitability level (694.67ha) and approximately 8,377.03ha under the S2 suitability level. The proposal of dragon fruit (6,773.08 ha) under the district's plan should be shifted to annual crops, other fruit trees (mango and grapefruit, etc.), and grass for livestock. In terms of the perennial crop, the majority were cultivated under Suitability level (S3) and Not suitability level (N); therefore, we propose keeping these crops growing under S3, and the remaining area (12,302.49ha) under N level should be converted to afforestation and soil improvement plants.

Conclusions
The analytical procedures developed in the five specific steps for assessing land suitability were used to develop an effective use of agricultural land in Ham Thuan Bac district, which had an area of 61126.09ha agricultural land and unused land with 11 common land-use types, with rice (21% of total agricultural land) and dragon fruit (32%) as the two main crops. Two important groups of land attributes for land suitability analysis including inherent qualities of soils (soil attributes) and external characteristics (topographic elements, and management) were used. Soil attributes included three factors: soiltype consisting of 17 soil types (in 8 soil groups), soil thickness consisting of 6 levels, and soil texture consisting of 4 levels. Slope characteristics consisting of 6 levels. The management characteristic is irrigation consisting of 3 regimes.
As a result of land mapping, based on the land characteristics, 107 LMUs were obtained over the entire agricultural and unused land of the study area. The majority belonged to Ferralsols (FA) groups (53 LMUs -52.24%), followed by Acrisols (AC) (17 LMUs -18.51%), Fluvisols (FL) (18 LMUs -12.37%), Arenosols (AR) (6 LUMs -10.15%), and the rest are other soil groups. The combination between characteristics of 107 LMUs and land use requirements were employed to evaluate current land use in the study area from suitable (S1, S2, and S3) to not suitable (N). The land area of LUTs of annual crops was mainly found at the S3 suitability level, where, for the current cultivated rice area, 23.29% of the area was unsuitable (N), and 67.75% was marginally suitable (S3). Meanwhile in terms of perennial trees, except for dragon fruits which were mainly cultivated at the S2 suitability level (78.28%), the other perennial LUTs (cashew, rubber, coffee, and other fruit trees) were cultivated with S3 and N levels, in which, the unsuitable land area was 65.13%, 44.47%, 85.61%, and 75.42%, respectively (in the total area of these LUTs).
To meet the requirements of sustainable land use, the following orientations have been proposed: (i) maintain the land areas which have High suitability (S1) and Moderate suitability (S2) levels; (ii) renovate or convert to other suitable plants/afforestation for Marginal suitable (S3) land areas; and (iii) convert to afforestation or use plants can improve soil quality for Not suitable (N) land areas. Therefore, for mainstay LUTs, the suggestions are to keep 8,550 hectares of land for rice with land suitability of S1, S2, and a part of the S3 levels which can be irrigated; To fit market demand, it is recommended to cultivate dragon fruit under the S1 suitability level (694.67ha) and approximately 8,377.03ha under the S2 suitability level; and the remaining areas, with S3 and N suitability levels for rice; and S2, S3, or N levels for dragon fruit, will be converted to other annual crops, other fruit trees, or grass for livestock.